The study of the morphology of living organisms is essential to understand
their evolution and diversity. This study aims to determine the importance
of climatic gradients in the diversity of pollen morphotypes using Atriplex halimus L.
(Amaranthaceae) as a model species. Atriplex halimus is a perennial shrubby plant,
polymorphic and very resistant to severe environmental conditions. In seven
bioclimatic zones, ranging from mesic conditions in the north to
hot–hyperarid in the south, pollen samples were collected from 49 sites,
with seven accessions per bioclimate. Under a light microscope, pollen grains
were selected and analyzed from three anthers of different flowers. Besides
the usual pollen grain types, some previously unknown morphotypes, such as
sulcate, triangular, and ovoid, were observed and described at the different
climatic zones. A total of 10 pollen grain shapes were quantified and
discussed following their specific occurrences within different climatic
zones. Occurrence frequencies of different pollen shapes ranged between 0 %
and 85.7 %, where the pantoporate spheroidal was the most widespread in
all climatic zones, especially in the desert climate (85.7 %). Five
pollen types occurred exclusively once per climate zone. The pantoporate
prolate spheroidal in mesomediterranean climate with a long dry season
(28.6 %), sulcate in the xerothermomediterranean climate (14.3%),
pantoporate subtriangular in the subdesert climate with a short dry season
(14.3 %), and pantoporate subprolate and boat-shaped in a subdesert
climate with a long dry season with 14.3 % for each. Our findings help to
understand the evolutionary effects of climate gradients on pollen
morphology and variability in arid and desert areas and point towards a
high degree of specialization in order to maximize trade-offs between
pollination efficiency and protection of pollen grains from dehydration.
Introduction
Pollen grain studies need to cover all phenotypical aspects (e.g., apertures,
shapes, pores, dimensions, colors), polarity traits, and also
cytoplasmic characteristics in order to efficiently resolve current
palynological problems (species and varietal identification in the fields of
botany, paleobotany, and forensic palynology) and thus advance our
understanding of the biology of pollination (Muller, 1979; Prieu, 2015).
Much scientific research has integrated pollen morphological
diversity with the taxonomy of angiosperms (Mallick, 2019; Prieu et al.,
2019). Pollen is involved in reproduction and is therefore subject to strong
selection pressures, which along with the environmental constraints influence
its morphology in both the short and long term (Young and Stanton, 1990a;
Prieu, 2015). Some environmental and/or biotic stresses can affect the
availability of resources to plants and thus reduce pollen production as a
result of changes in the number of flowers and pollen production per flower.
These stresses can be related to soil fertility (Lau et al., 1995; Havens et
al., 1995), previous production of fruits and seeds (Delph, 1990; Young and
Stanton, 1990a; Stephenson et al., 1994), mycorrhizal infection and
effectiveness (Lau et al., 1995), and herbivory intensity (Quesada et al.,
1995).
The current study focused on morphological diversity of pollen grains of the
Mediterranean saltbush Atriplex halimus L. (Amaranthaceae) in steppe rangelands of Algeria,
where the species stretches over about 10 000 km2 in the form
of halophytic steppe vegetation (Macheroum and Kadik, 2015). In general, North
African steppes in semiarid and arid regions are in a serious state of
degradation due to the continuous deterioration of vegetation cover
undergoing severe environmental forcings viz. drought and soil salinization,
combined with serious human pressures, mainly overgrazing and cultivation of
nonagricultural lands (Neffar et al., 2013, 2018; Kouba et al., 2020).
Atriplex halimus is an interesting species, because it divulges many characteristics of the
reproductive system at the scale of the plant (Talamali et al., 2006) and is
characterized by a significant diversity, on morphological and genetic
scales (Ortiz-Dorda et al., 2005; Benzarti et al., 2013). The species adapts
well to difficult environmental conditions with a high tolerance to salinity
and drought (Neffar et al., 2016; Chenchouni, 2017). It is native in Europe,
North Africa, and the Middle East, where it is used as an important plant for
reclamation of degraded rangelands in arid and semiarid regions due to its
high foraging value for livestock. The species has several medicinal
virtues, for instance leaves can be used to treat heart diseases, diabetes,
and rheumatism; it can also be used for phytoremediation and
phyto-desalination of road runoff (Walker et al., 2014; Suaire et al.,
2016).
According to the angiosperm phylogeny group (Müller and Borsch, 2005; Angiosperm Phylogeny Group, 2009), the pollen of Amaranthaceae species, especially A. halimus, is little
studied. The available data, so far, included some inputs about number of
pores, pollen grain diameter, and spinule and puncta densities (Hao et al.,
1989; Flores Olvera, 1992). Mulder (1999) proposed a key to classify the
Mediterranean types of pollen shapes in Amaranthaceae (formerly known as
Chenopodiaceae) inhabiting drylands. Several studies suggested that
phenotypic changes in plant organs can be fast in nature (Reznick and
Ghalambor, 2001; Uyeda et al., 2011). We assume that the unusual
morphological shapes are a priori due to plant adaptation to certain factors
such as temperature, intra- and/or interspecific competition, phenotypic
plasticity, and mutations (Delph, 1990). The latter may be at the origin of
the appearance of certain types of pollen (Young and Stanton, 1990b).
The current survey aimed at studying pollen morphological diversity of A. halimus in
arid and semiarid steppe rangelands of NE Algeria. It analyzed new pollen
types encountered at the different bioclimatic zones of this region. The
analysis included qualitative and quantitative comparisons of
similarity and dissimilarity of pollen types between the climate regions
surveyed.
Materials and methodsStudy area
This study was carried out in halophytic steppe rangelands, mostly dominated
by A. halimus, growing wild in the region of Tébessa (34∘15′ to
35∘45′ N, 7∘30′ to 8∘30′ E) located in
northeastern Algeria (Fig. 1). With an area of about 14 000 km2, this region rises about 960 m above sea level and is
naturally connected with the immense steppe expanse of the country. It
belongs to the semiarid bioclimatic zone of North Africa, located between
the two Atlas mountain ranges, namely the Tellian mountains in the north and Saharan mountains in the
south. A large climatic gradient exists from north to south over Tébessa
region (Fig. 1), with more or less mesic conditions in the north and
hot-hyperarid in the south (Mekahlia et al., 2013).
Elevation map of Algeria (a) and map of bioclimate zones of
Tébessa (c) with its legend (b), displaying locations in the
study area and distribution of steppe rangelands of Atriplex halimus (forest-green color in
right map) in the region of Tébessa, northeastern Algeria. The symbols “+”
and “–” associated with climate codes in the map legend refer to short and long
dry seasons of climates, respectively.
Based on long-term meteorological data (1972–2015) of Tébessa weather
station, the average annual temperature was 15.5 ∘C and the
average annual rainfall is about 371.2 mm. July was the hottest month with
an average temperature of 27.3 ∘C, while the lowest temperature
(6.4 ∘C) was recorded during January. September was the rainiest
month (average monthly rainfall = 43.1 mm), whereas the driest month was
July with an average of 14.37 mm (Fig. 2; Supplement, Table S1). The region of
Tébessa is characterized by a semiarid rangeland vegetation type,
principally dominated by Stipa tenacissima, Artemisia herba-alba, and Atriplex halimus. Lands are used mainly for the cultivation
of rainfed cereal crops (durum, barley, and bread wheats) and livestock
grazing (Djellab et al., 2019). Following Köppen's climatic
classification, the arid climate of the region characterized either a cold
steppe “BSk” or hot desert “BWh”, where precipitation deficit exceeded 800 mm yr-1 and limited the climatic net primary production (Table A1 in Appendix A).
Ombrothermic diagrams of Gaussen and Bagnouls applied for the
climatic zones of the region of Tébessa, northeastern Algeria.
Pollen sampling
Sampling was carried out in seven bioclimatic zones that characterize the
region of Tébessa, ranging from mesomediterranean in the north to desert
in the south (Fig. 1). Within each of the seven climatic zones studied,
seven accessions of A. halimus were randomly sampled to study pollen morphology. A set
of 20 pollen grains from each accession was selected from three anthers of
different flowers that were collected from leafy and inflorescent twigs of
native and wild-grown A. halimus plants. A total of 49 sites were sampled and analyzed
(Table A2).
Pollen visualization
Anthers from each sample and of each accession were placed into drops of
H2SO4 (95 %) on microscopic glass slides. The slides were heated
over a candle flame for 10 s to mollify the anthers, release the
pollen grains from anthers, and dissolve extra organic material on pollen
grain surfaces (Grímsson et al., 2018). Randomly selected, 20 pollen
grains from each anther were then transferred onto new slides for
identification under a light microscope with 400× magnification. By
referring to illustrated pollen terminology (Halbritter et al., 2018) and
a glossary of pollen and spore terminology (Punt et al., 2007), types and
shapes of pollen grains were sorted and characterized. We also used
morphological pollen determination keys following the example of Erdtman (1952), who categorized eight shape classes based on the ratio of polar
axis (PA) to equatorial diameter (ED). Based on the equatorial view, the
ratio between the PA and ED, multiplying by 100 gives an accurate indication
of pollen shape; accordingly, pollen types have been classified as shown in Table 1.
Pollen shape classes and suggested relationships between polar axis
(PA) and equatorial diameter (ED) (Erdtman, 1952).
For some pollen grain types, such as “pantoporate”, it is not
possible to distinguish where the proximal and distal poles are, and, thus,
the PA and ED, the ratio of larger dimension to smaller dimension was employed
instead of PA / ED. Observations revealed the existence of pantoporate
spheroidal globular types (usual pollen grains in Amaranthaceae) as well as
exceptional shapes found at certain study sites such as triangular,
boat-shaped, and sulcate (Fig. 3).
Light micrographs showing equatorial views of pollen grains of the
Atriplex halimus collected from arid and semiarid steppe rangelands of the region of
Tébessa, northeastern Algeria. Pollen shapes identified: (A) pantoporate
spheroidal, (B) pantoporate prolate spheroidal, (C) pantoporate triangular
pyramid, (D) pantoporate oblate spheroidal, (E) pantoporate subtriangular, (F) pantoporate suboblate, (G) sulcate, (H) pantoaperturate spheroidal, (I) boat-shaped, and (J) pantoporate subprolate. Scale bar = 10 µm.
Data analysis
The number of pollen type occurrences per climatic zone was computed and
displayed as a heatmap with two-way clustering analysis. Based on a
contingency table summarizing the number of pollen types present at each
climatic zone, a correspondence analysis (CA) was performed in order to
distinguish the pollen types that characterize each zone and those that are
common among them. The variation in occurrence odds of each pollen grain
shape among the bioclimatic zones was tested using a generalized linear model
(GLM) with a binomial distribution error and logit link. In addition, the
effects of latitude, longitude, and altitude on the occurrence of each pollen
grain shape were tested using a generalized linear mixed-effect model (GLMM),
where geo-coordinates (latitude, longitude, and altitude) were the fixed
effects and the bioclimatic zones were considered the random effect.
Similarity of pollen morphotypes between climatic zones was analyzed
qualitatively and quantitatively using the Jaccard coefficient and Bray–Curtis
distance, respectively. Shared and zone-specific pollen shapes among the
climatic zones were displayed using a seven-set Venn diagram. Statistical
analyses were carried out using the R packages d3heatmap, nlme, and
venn.
ResultsSpatial occurrences and distribution of pollen grains
Pollen occurrence frequency per climatic zone varied between 0 % and 85.7 %
(Fig. 4). The most common pollen type in A. halimus among the climatic zones studied
was the pantoporate spheroidal type that totaled 85.7 % of pollen
occurrences at the desert climate. The lowest value of occurrence
frequency of this type was recorded at Sb.Des- and Ms.Med+
with 28.6 %, followed by Th.Med- and Sb.Des+ zones with
42.9 %, and then Th.Med+ and X.T.Med zones with 57.1 %. As for the
pantoaperturate spheroidal type, which was present in all studied climatic
zones except Th.Med+ and desert, its occurrence was the highest at
Th.Med- where it totaled 57.1 %. Occurrence frequencies of
this pollen type were similar at Ms.Med+ and Sb.Des-
(42.9 %) and at X.T.Med and Sb.Des+ (14.3 %). At Sb.Des+
pollen grains with pantoaperturate spheroidal, pantoporate oblate
spheroidal, pantoporate subprolate, boat-shaped, and pantoporate suboblate
types have the same occurrence frequency (14.3 %). It is worth
mentioning that five pollen types occurred exclusively in one climate zone,
namely the pantoporate prolate spheroidal type in Ms.Med+ (28.6 %), the
sulcate type in X.T.Med (14.3 %) and pantoporate subtriangular type in
Sb.Des- (14.3 %), and the pantoporate subprolate and
boat-shaped types in Sb.Des+ with 14.3 % for each (Fig. 4).
Heatmap with two-way clustering analysis of pollen grain types of
Atriplex halimus collected from seven bioclimatic zones in steppe rangelands of northeastern
Algeria. Clustering was performed using absolute occurrences of pollen
shapes, whereas figures associated with color intensity and displayed in the
plot are relative frequency in percent. (See Fig. 1 for codes of the
bioclimatic zones studied.)
Spatial variation in pollen grain occurrences
Results of GLMs testing the variation in pollen shape occurrences
(presence or absence of data) among the bioclimatic zones showed that there was no
significant difference in occurrence odds of all pollen aspects except for
the pantoaperturate spheroidal type whose occurrences varied significantly
between climatic regions (χ2=14.39, p=0.026) (Table 2).
Furthermore, the GLMM testing effects of latitude, longitude, and altitude
on the spatial occurrence of each pollen grain shape revealed that latitude
had a negative effect on occurrence odds of all pollen aspects except for
the pantoaperturate spheroidal type that was positive (GLMM: t=1.581,
p=0.122); however all these effects were not statistically significant
except for the pantoporate triangular pyramid type (t=-2.41, p=0.021)
(Table 3). Spatial occurrence of the sulcate type was deemed negatively
influenced by longitude (t=-2.396, p=0.022) and positively by altitude
(t=2.047, p=0.047). Thus, the linear model predicting occurrence odds of
the previous two pollen types following geo-coordinates in northeastern
Algeria is expressed as follows:
pantoporatetriangularpyramid=(-0.161×latitude)+(0.151×longitude)+(2.69×10-4×altitude)+4.296sulcate=(-0.006×latitude)-(0.167×longitude)+(2.06×10-4×altitude)+1.38
with latitude and
longitude expressed in decimal units and altitude in meters.
Generalized linear models (binomial distribution + logit link)
testing the variation in each pollen grain type of Atriplex halimus among seven bioclimatic
zones in steppes of northeastern Algeria (Df: degrees of freedom; χ2: chi-square value of likelihood ratio (LR) test).
For the rest of pollen types, GLMMs indicated that effects of latitude,
longitude, and altitude on the variation in spatial occurrences were not
significant (Table 3).
Generalized linear mixed models (GLMMs) testing the effects of
latitude, longitude, and altitude on the occurrence of different pollen types
of Atriplex halimus in steppe rangelands of northeastern Algeria.
The correspondence analysis (CA) determined both characteristic and common
pollen types of different climatic zones surveyed in NE Algeria (Fig. 5).
The two axes of CA captured 57.48 % of the total variance. The pantoporate
prolate spheroidal type was deemed to characterize the mesomediterranean climate
with a long dry season and the pantoaperturate spheroidal type characterized the
thermomediterranean climate with a short dry season. Pantoporate
triangular pyramid and pantoporate subtriangular were the typical pollen
types in the subdesert climate with a short dry season. The subdesert
climate with a long dry season was characterized by pantoporate subprolate
and boat-shaped types. Pantoporate oblate spheroidal, pantoporate suboblate,
pantoporate spheroidal, and sulcate constituted a cluster that characterized
mainly thermomediterranean with a long dry season, xerothermomediterranean, and
desert climates.
Biplot (F1 × F2) of the factorial correspondence analysis
(CA) showing the distribution of pollen grain types of Atriplex halimus among seven
bioclimatic zones in steppe rangelands of northeastern Algeria. (See Fig. 1 for codes of the bioclimatic zones studied.)
Diversity and distribution of pollen grains
Following the Venn diagram, the pantaporate spheroidal type was the common
pollen morphotype of A. halimus that was observed in all climatic zones (Fig. 6).
Pantoaperturate spheroidal was common between five climatic zones,
pantoporate suboblate and pantoporate oblate spheroidal were shared between
three zones, and pantoporate triangular pyramid occurred exclusively in two
zones. Three pollen types were exclusively specific to one climate zone:
pantoporate prolate spheroidal type in Ms.Med+, sulcate in X.T.Med, and
pantoporate subtriangular in Sb.Des-. The Sb.Des+ zone had
two exclusive pollen grain types (pantoporate subprolate and pantoporate
triangular pyramid).
Seven-set Venn diagram displaying the occurrence of different
pollen types of Atriplex halimus among climatic zones in steppe rangelands of northeastern
Algeria. (See Fig. 1 for codes of the bioclimatic zones studied.) Values
between square brackets are the total number of pollen types “S” per
climate.
Qualitative and quantitative similarity analysis
Based on Bray–Curtis distances (quantitative similarity), 11 out 21 pairwise
comparisons revealed high similarity (>50 %) (Fig. 7). The
highest similarity scores (71 %) were observed for the pairs
Ms.Med+ and Th.Med-, Ms.Med+ and Sb.Des-,
Th.Med- and Sb.Des-, Th.Med+ and X.T.Med, and
Th.Med+ and desert. The lowest quantitative similarity (29 %) of pollen
shapes climates zones was recorded between for the pairs
Ms.Med+ and Th.Med+, Ms.Med+ and desert, and
Sb.Des- and desert. The qualitative similarity analysis indicated
low similarity between climatic zones where only 23.8 % of computed
similarities showed values of Jaccard index greater than 50 %. The highest
similarity scores (Jaccard = 66.7 %) were recorded between relatively
mesic climates of the region, i.e., Ms.Med+ and Th.Med-, while the
lowest similarity (16.7 %) was obtained between Ms.Med+ and
Th.Med+.
Matrix of similarity of pollen grain types between seven climatic
zones in Algeria. Values of similarity are displayed in color intensity
proportionally to Bray and Curtis distance (above diagonal) and Jaccard'
similarity coefficient (below diagonal). (See Fig. 1 for codes of the
bioclimatic zones studied.)
Discussion
This essay fundamentally treated the questions related to morphological
diversity of pollen grains in different bioclimatic zones of Algeria (North
Africa). The study investigated the reasons that led to the appearance of
new pollen types in A. halimus collected from arid and semiarid steppe rangelands of
Algeria. It is worth mentioning that studying pollen diversity
contributes greatly to the enrichment of pollen libraries and thus helps to
understand the mechanisms of taxonomic and functional speciation.
Patterns of pollen grains observed
The most frequent pollen type in A. halimus is pantoporate spheroidal which was
encountered at all climatic zones studied. However, other pollen aspects and
types of unprecedented sizes and shapes, such as triangular, boat-shaped, and
sulcate with two tight poles, as well as different colors like orange,
purple, pink, and yellow, have been recorded in all climate zones.
This variation in pollen colors can be explained by chemical composition.
Indeed, chemical studies of pollen pigments have shown that there are two
main classes of components that determine pollen color: flavonoids and
carotenoids, and there can be intraspecific variations in terms of pollen
colors (Kearns and Inouye, 1993). Our findings highlighted the existence of
intraplant polymorphism, since the exceptional pollen types were observed
side by side with the common interplant and intersite shapes. The observed
pollen grains of A. halimus in northeastern Algeria were of different colors and sizes
(small, large, and intermediate), which corroborates with the results of
Talamali et al. (2007), who studied two A. halimus populations in Tunisia, one from the
region of Tadjerouine, located near our study area, where in addition to the
three pollen sizes described above, orange- and yellow-colored pollen
grains have been reported. The pantoaperturate spheroidal type, very close
to the pantoporate spheroidal type, was also the most commonly observed with
the latter.
Factors influencing the shape of pollen grains
Regarding the intraspecific pollen size variation, several studies have
addressed the question with hypotheses in order to explain these
evolutionary changes (Ejsmond et al., 2011). Beaulieu et al. (2008) proposed that polyploidy and/or genome sizes are predictors of larger
sizes of pollen grains. According to Amer and Amany (2014), there is a solid
relationship between polyploidy and pollen morphological variation in A. halimus,
where the most touched morphological characters are exine features, pore
membrane, number of pores, pore shape, spinule shape, and pore margin.
However, Knight et al. (2010) revealed that there was no relationship
between pollen size and genome size through controlling the phylogenetic
history of 464 plant species.
Furthermore, abiotic stresses affect pollen development at almost all
ecozones worldwide (Firon et al., 2012; Bokszczanin and Fragkostefanakis, 2013; Müller
and Rieu, 2016; García et al., 2017). In fact, pollen grain size may
vary depending on a bunch of environmental and biotic variables controlling
the growing conditions (Muller, 1979; Young and Stanton, 1990a; Stephenson
et al., 1994; Torres, 2000). Among these conditions, the air temperature and
moisture are the most crucial factors (Ejsmond et al., 2011; Firon et al.,
2012). Indeed, Ejsmond et al. (2011) stressed that the number of pollen
produced as well as the optimization of its size are closely related to the
ambient temperature, but the latter does not significantly affect its shape.
Concerning the effects of moisture on pollen, pollen grain walls (intine and
exine) can cope with changes in volume due to up-take or loss of water; they
lose water, decreasing in volume, when kept in a dry environment, but gain
water, increasing in volume, when kept under humid conditions (Pacini, 1990;
Pacini and Franchi, 2020). On the other hand, the plasma membrane plays a
very important role in regulating the inflow and outflow of water. Its
structure changes according to the water content of the grain (Platt-Aloia
et al., 1986; Tiwari et al., 1990). Different responses of the plasma
membrane to relative moisture have been detected in plants of the same group
but living in different habitats (Chaudhury and Shivanna, 1987).
Before the opening of the anthers, pollen grains endure a significant
dehydration and generally lose about 15 %–35 % of cytoplasmic water.
In addition, under certain conditions, this loss of water leads to a reduction
in pollen grain volume and the wall folds back along the apertures (Nepi et
al., 2001). Pollen grains are rehydrated once they reach a compatible stigma
and increase in volume, leading to harmomegathy, which means changes in
pollen volume accommodation in response to water loss (Wodehouse, 1965).
According to Volkova et al. (2013), these harmonic changes occur when the
cytoplasm and pollen grain walls collaborate to maintain protoplast
viability, despite changes in shape and volume. This process is facilitated
by pollen wall grooves (colpi) that facilitate the change in pollen grain
shape after water loss.
Variations in the composition, organization, and shape of pollen grains are
mainly due to harmomegathic stress (Muller, 1979; Payne, 1981). This could
explain the pantoporate oblate spheroidal types of pollen that were observed
in climatic zones 3, 6, and 7. In light of these results, it can be argued
that the climate aridity and drought that characterize climatic zones 5 and
6 could influence the new pollen morphotypes (i.e., sulcate, triangular, and
boat-shaped) found in these zones. Katifori et al. (2010) stipulate that in
dry environments, the pollen wall has the ability to fold back on itself in
order to prevent further losses of water, which is in agreement with the
work of Muller (1979). These results were affirmed by Volkova et al. (2013),
who testify that the pollen grains have developed the ability to bend in
order to avoid dehydration, which could explain the appearance of the
sulcate and pantoporate subtriangular types in climatic zones 4 and 5 of
the study area. Moreover, plants coexisting in the same habitat with limited
resources and/or under extreme conditions compete for the resources
necessary for their growth. This competition also concerns pollen, where it
can affect pollen phenotypes of offspring (Snow and Mazer, 1988) and pollen
performance (Schlichting, 1986).
Furthermore, the change of pollination mode can influence pollen shape and
size (Torres, 2000). In the same sense, Edlund et al. (2004) suggest that
pollen grain size is the result of biotic and abiotic preference in
pollinators. This preference is linked to diverse traits of pollen of
angiosperms, including the diversity of nutrients available for pollinators
(Roulston et al., 2000), the diversity of flavors (Dobson and Bergström,
2000), and pigments (Lunau, 2000). Regarding pollen colors, both carotenoids
and flavonoids may cause a yellow pollen color. According to Stanley
and Linskens (1974), pollen grains contain representatives of the colorless
and yellow flavonoids; while most flavones and flavonols combine to yield
yellow-colored pollen, some pollen is red or blue. Anthocyanins are
generally the source of these colors. The amount of flavonoid, its location,
and the presence of other pigments all contribute to the visible color of
pollen, and the appearance of its extract. Small quantities of carotenoids
can drastically modify the apparent color of pollen flavonoids. Identical
flavonoids may be present in two species but because of variation in
concentration and presence of co-pigments, the pollens may appear different
in color. These diverse traits can also explain the different pollen colors
(orange, yellow, pink, and purple) observed in this study.
Moreover, it is important to mention that some flowers of angiosperms
produce sterile pollen to attract and reward pollinators. For example,
dimorphic anthers of Lagerstroemia indica produce fertile blue pollen and nourishing sterile
yellow pollen. Although the nourishing pollen can germinate, its tubes never
reach the style (Pacini and Bellani, 1986; Pacini and Franchi, 2020).
Other factors may lead to pollen phenotypic changes. Among these factors,
altitude plays an important role in pollen morphological diversity.
According to Prieu (2015), pollen grains with four apertures are more
frequent at high elevations, and pollen grains with five apertures dominate
at low elevations. The highest elevation in the study area is 1090 m
recorded in zone 4 where the sulcate and pantoporate subprolate types were
observed. In some plants, elevation affects pollination efficiency, which
decreases with the increase in altitude (Till-Bottraud et al., 1999). This can be
explained by the fact that pollinators are rarer at high altitudes, but this
incites production of pollen that survives for a long time and thus reduces the
vagaries of pollination (Prieu, 2015).
Heteromorphism and polymorphism
Pollen heteromorphism, i.e., the production of several fertile pollen formed by a
single plant, is seen in about one in three families of angiosperms (Till et
al., 1989). This study showed that some individuals of A. halimus produced different
types of pollen “heteromorphism”. The production of several types of
pollen can be advantageous in the long term, because each type is adapted to
particular environmental conditions. This explains the high level of
heteromorphism in flowering plants (Mignot et al., 1994). In a study
involving 200 species (Mignot, 1995), no purely polymorphic species in
regards to pollen has been detected. In addition, pollen variability can be
influenced by the efficiency of transport by different pollinating insect
species that can lead to evolutionary stability of pollen heteromorphism
(Ejsmond et al., 2011).
Genetic aspects, phenotypic plasticity, and genotype–environment interaction
It is important to consider the genetic features to understand the
appearance of certain shapes of pollen. In natural populations, genetic
variations in pollen performance may result from mutations, gene flows among
populations, and environmental heterogeneity within populations (Schlichting
et al., 1990; Charlesworth and Charlesworth, 1992). Our results suggest the
existence of intraindividual genetic variations, which are probably related
to somatic mutations caused by the indefinite plant growth (Hallé,
2015). On the other hand, it seems unclear whether mutations are the only
causes that could explain such a diversity of pollen. This leads us to
suggest that these mutations can be, with the factors mentioned above, at
the origin of a certain new type observed in this study such as the
pantoporate prolate spheroidal type in the mesomediterranean climate with a
long dry season, pantoporate subprolate grains in the subdesert climate with a
short dry season, and pantoporate suboblate grains in
xerothermomediterranean, thermomediterranean, and subdesert climates with a
long dry season.
In addition, certain nongenetic pollen variations may correspond to phenotypic
plasticity, which can be defined as the change of a character in response to
a variation in the environment for a given genotype (Talamali et al., 2006;
Lefèvre et al., 2015). This can explain the occurrence of certain pollen
shapes as a response to environmental stresses and hazards. The triangular
pyramid type is among these types that were recorded in the
thermomediterranean (long dry season) and subdesert (short dry season)
climates. The same is true for the boat-shaped type which was recorded in the
subdesert (long dry season) climate. Accordingly, the following question
arises: why did the majority of the observed pollen keep its usual shape?
This could be explained by homeostasis, where a genotype maintains a stable
character when the environment varies. There are three possible situations
for a character to change following genotype and/or a range of environmental
conditions: genetically non-variable plasticity (effect of the environment),
absence of plasticity (no effect of the environment), or genetically variable
plasticity (genotype × environment interaction). In the case of
variable plasticity, the term “ecovalence” is also used, which refers to
the contribution of a genotype to the genotype × environment
interaction (Lefèvre et al., 2015).
Conclusion
The current study provided – for the first time – an extensive and
comprehensive analysis of diversity and variations in pollen grain shapes of
Atriplex halimus growing wild across a large climatic gradient in the steppes of Algeria and
North Africa. New and previously unknown pollen types, with unique
morphological characteristics such as grain size and shape were observed and
described. The statistical modeling approach accompanied by qualitative
and quantitative similarity analysis assisted in understanding spatial
variations in pollen grains encountered in different bioclimatic zones in
Algeria. Pollen morphological diversity is mainly attributed to abiotic
factors such as air temperature that influence both size and shape of pollen
grain, giving it the ability to fold back on itself to decrease dehydration.
Other factors such as animal pollinators and altitude also play a role in
pollen morphological variability. Addressing ecological variables behind
pollen variability may explain morphophysiological adaptations of
individuals and populations to climate change and unfavorable conditions.
Carrying out genomic studies appears to be necessary in order to better
understand the diversity of pollen recorded in this work. This can also be
applied to other plant species of economic and medicinal interest.
Climatic characteristics of the zones in which arid and
semiarid steppe rangelands of Atriplex halimus were studied in the region of Tébessa, NE
Algeria.
a See Fig. 1 for codes of the bioclimatic zones studied.
b NPP: net primary production, in grams of dry matter per square meter per year,
is precipitation limited.
Geo-coordinates (latitude and longitude) and elevation of
sample points used in collection pollen grains of Atriplex halimus in the region of Tébessa,
NE Algeria.
Climatic zonesLocalitySiteLatitudeLongitudeElevationno.(north)(east)(m a.s.l.)Mesomediterranean (long dry season)Laouinet135∘53′29′′07∘52′53′′672(code: Ms.Med+)Laouinet235∘52′40′′07∘52′52′′654Boukhadra335∘44′02′′07∘56′34′′694Boukhadra435∘42′04′′08∘00′02′′778Laouinet535∘53′46′′07∘54′41′′651Laouinet635∘53′56′′08∘02′34′′594Morsott735∘40′41′′07∘59′57′′752Thermomediterranean (short dry season)Boulhaf Dyr835∘28′08′′08∘05′00′′812(code: Th.Med-)Tébessa935∘26′08′′08∘03′36′′795Hammamet1035∘27′39′′07∘58′32′′826Tébessa1135∘25′29′′08∘07′06′′813Tébessa1235∘23′29′′08∘10′05′′842Bekkaria1335∘21′48′′08∘14′23′′909Tébessa1435∘25′06′′08∘08′26′′814Thermomediterranean (long dry season)Bekkaria1535∘21′55′′08∘11′16′′922(code: Th.Med+)Bekkaria1635∘20′57′′08∘11′34′′954El Ma Labiodh1735∘07′58′′08∘12′02′′988Safsaf El Ouesra1834∘57′38′′08∘12′37′′881Safsaf El Ouesra1934∘56′36′′08∘10′56′′905Safsaf El Ouesra2034∘56′24′′08∘10′36′′910Safsaf El Ouesra2135∘02′06′′08∘14′49′′929XerothermomediterraneanTlidjene2235∘09′05”07∘46′02′′1060(code: X.T.Med)Tlidjene2335∘05′02′′07∘44′57′′985Tlidjene2435∘02′39′′07∘46′13′′1020Stah Guentis2535∘01′40′′07∘20′08′′1060Stah Guentis2634∘59′51′′07∘17′58′′984Stah Guentis2735∘08′29′′07∘27′38′′1090Stah Guentis2834∘58′39′′07∘17′16′′1070Subdesert (short dry season)Bir El Ater2934∘45′01′′08∘05′22′′862(code: Sb.Des-)Bir El Ater3034∘40′10′′07∘55′23′′630Bir El Ater3134∘46′28′′08∘03′52′′838Bir El Ater3234∘44′16′′08∘11′05′′805Bir El Ater3334∘44′37′′08∘ 13′32′′729Bir El Ater3434∘44′57′′08∘14′29′′710Bir El Ater3534∘46′06′′08∘ 16′48′′701Subdesert (long dry season)Bir El Ater3634∘38′51′′07∘52′43′′590(code: Sb.Des+)Bir El Ater3734∘ 38′45′′07∘ 52′14′′584Bir El Ater3834∘38′33′′07∘52′24′′570Bir El Ater3934∘38′19′′07∘50′38′′557Bir El Ater4034∘38′05′′07∘50′04′′544Bir El Ater4134∘37′49′′07∘49′12′′534Bir El Ater4234∘36′14′′07∘45′41′′485DesertFerkane4334∘33′31′′07∘23′44′′241(code: Desert)Ferkane4434∘33′22′′07∘23′16′′224Negrine4534∘30′51′′07∘36′47′′367Negrine4634∘33′54′′07∘42′06′′432Negrine4734∘34′14′′07∘42′53′′447Negrine4834∘34′32′′07∘43′23′′465Negrine4934∘30′26′′07∘35′46′′368Data availability
The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.
The supplement related to this article is available online at: https://doi.org/10.5194/we-20-19-2020-supplement.
Author contributions
HF and SM conceived the study. HF conducted
fieldwork, collected plant samples, carried out laboratory measurements, and
compiled data. BH contributed to data collection. HC analyzed data and
designed the paper. HF, HC, and AD wrote and revised the manuscript. All
authors read and approved the final version of the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Review statement
This paper was edited by Daniel Montesinos and reviewed by António Xavier Pereira Coutinho and two anonymous referees.
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