Dispersal capacities of pollen, seeds and spores: insights from comparative analyses of spatial genetic structures in bryophytes and spermatophytes

Introduction

The dramatic fluctuations of climate conditions since the late Tertiary era have resulted in major species range shifts. These movements were conditioned by geographic barriers and species dispersal capacities. In land plants, gene flow occurs through the movement of male gametes (sperm cells, pollen grains), which carry nDNA, and diaspores (spores, seeds), which carry both cpDNA and nDNA, making them an ideal model to compare the imprints of past climate change on the spatial genetic structures of different genomic compartments. Based on a meta-analysis of cpDNA and nDNA sequence data in western Europe, we test the hypotheses that nDNA genetic structures are similar in bryophytes and spermatophytes due to the similar size of spores and pollen grains, whereas genetic structures derived from the analysis of cpDNA are significantly stronger in spermatophytes than in bryophytes due to the substantially larger size of seeds as compared to spores.

Methods

Sequence data at 1-4 loci were retrieved for 11 bryophyte and 17 spermatophyte species across their entire European range. Genetic structures between and within southern and northern populations were analyzed through F and N statistics and Mantel tests.

Results and discussion

Gst and Nst between southern and northern Europe derived from cpDNA were significantly higher, and the proportion of significant tests was higher in spermatophytes than in bryophytes. This suggests that in the latter, migrations across mountain ranges were sufficient to maintain a homogenous allelic structure across Europe, evidencing the minor role played by mountain ranges in bryophyte migrations. With nDNA, patterns of genetic structure did not significantly differ between bryophytes and spermatophytes, in line with the hypothesis that spores and pollen grains exhibit similar dispersal capacities due to their size similarity. Stronger levels of genetic differentiation between southern and northern Europe, and within southern Europe, in spermatophytes than in bryophytes, caused by higher long-distance dispersal capacities of spores as compared to seeds, may account for the strikingly higher levels of endemism in spermatophytes than in bryophytes in the Mediterranean biodiversity hotspot.

No borders during the post-glacial assembly of European bryophytes

Climatic fluctuations during the Last Glacial Maximum (LGM) exerted a profound influence on biodiversity patterns, but their impact on bryophytes, the second most diverse group of land plants, has been poorly documented. Approximate Bayesian computations based on coalescent simulations showed that the post-glacial assembly of European bryophytes involves a complex history from multiple sources. The contribution of allochthonous migrants was 95-100% of expanding populations in about half of the 15 investigated species, which is consistent with the globally balanced genetic diversities and extremely low divergence observed among biogeographical regions. Such a substantial contribution of allochthonous migrants in the post-glacial assembly of Europe is unparalleled in other plants and animals. The limited role of northern micro-refugia, which was unexpected based on bryophyte life-history traits, and of southern refugia, is consistent with recent palaeontological evidence that LGM climates in Eurasia were much colder and drier than what palaeoclimatic models predict.