Abstract
Differences in ploidy levels among different fern species have a vast influence on their mating system, their colonization ability and on the gene flow among populations. Differences in the colonization abilities of species with different ploidy levels are well known: tetraploids, in contrast to diploids, are able to undergo intra-gametophytic selfing. Because fertilization is a post-dispersal process in ferns, selfing results in better colonization abilities in tetraploids because of single spore colonization. Considerably less is known about the gene flow among populations of different ploidy levels. The present study examines two rare fern species that differ in ploidy. While it has already been confirmed that tetraploid species are better at colonizing, the present study focuses on the gene flow among existing populations. We analyzed the genetic structure of a set of populations in a 10×10 km study region using isoenzymes. Genetic variation in tetraploid species is distributed mainly among populations; the genetic distance between populations is correlated with the geographical distance, and larger populations host more genetic diversity than smaller populations. In the diploid species, most variability is partitioned within populations; the genetic distance is not related to geographic distance, and the genetic diversity of populations is not related to the population size. This suggests that in tetraploid species, which undergo selfing, gene flow is limited. In contrast, in the diploid species, which experience outcrossing, gene flow is extensive and the whole system behaves as one large population. Our results suggest that in ferns, the ability to colonize new habitats and the gene flow among existing populations are affected by the mating system.
Highlights
Gene flow is the successful movement of genes among populations by mating or by the migration of diaspores, and it is one of the key factors determining the spatial genetic structure of populations [1,2]
Our study addressed the following questions: 1) What is the distribution of genetic diversity within and among the populations of Asplenium adulterinum and A. cuneifolium?, 2) What is the intensity of gene flow among the populations? and 3) Do the patterns differ between species, and do they correspond to the expected difference in mating systems?
The present study revealed considerable differences in the genetic structure of two populations of rare fern species differing in ploidy level
Summary
Gene flow is the successful movement of genes among populations by mating or by the migration of diaspores, and it is one of the key factors determining the spatial genetic structure of populations [1,2]. Gene flow is usually considered beneficial for population survival, preventing inbreeding depression and the loss of genetic variation in small populations due to genetic drift [3]. Gene flow can be detrimental for small populations because it prevents differentiation through the local adaptations of populations in different extreme conditions and reduces individual fitness through outbreeding depression [4]. The intensity of gene flow is, to a vast degree, influenced by the mating system of the species [5,6]. The effects of the mating system on the intensity of gene flow has been extensively studied in seed plants (meta-analysis in [6]), and it was shown that gene flow among populations increases with an increasing level of outcrossing [6]. Three types of fertilization occur in ferns [7]: (i) intra-gametophythic selfing (the fusion of sperm and egg from the same gametophyte resulting in a complete homozygote); (ii) inter-gametophytic selfing (the fusion of sperm and egg from different gametophytes derived from the same parental sporophyte, which is equivalent to selfing in seed plants); and (iii) outcrossing (the fusion of sperm and egg from gametophytes derived from spores of different sporophytes)
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