Balance between inbreeding and outcrossing in a nannandrous species, the moss Homalothecium lutescens

Abstract

Epiphytic dwarf males on the females present a possible solution to the problem of short fertilization distances in mosses. However, leptokurtic spore dispersal makes dwarf males likely to be closely related to their host shoot, with an accompanying risk of inbreeding. The capacity of a female to harbour a high number of different dwarf males suggests that there may be mechanisms in place that counteract inbreeding, such as polyandry and post-fertilization selection. We have genotyped sporophytes, female host shoots and dwarf males in four populations of the moss Homalothecium lutescens. We found no evidence of selective sporophyte abortion based on level of heterozygosity. The occurrence of entirely homozygous sporophytes together with significantly positive inbreeding coefficients in three of the populations (mean F IS between 0.48 and 0.64) suggest frequent mother-son mating events. However, 23% of all sampled sporophytes had a higher level of heterozygosity compared with the mean expected heterozygosity at the population level. Polyandry was frequent, on average 59% of the sporophytes on a female shoot were sired by distinct fathers. In conclusion, sporadic fertilizations by dwarf males originating from nonhost female shoots appear to counteract strong inbreeding.

Genetic Diversity and Population Structure in Bryophyte With Facultative Nannandry

“…H. Rob. have greatly extended our knowledge about nannandry in bryophytes ( Rosengren et al, 2013 , 2015 , 2016 ). This species is pleurocarpous, having sporophytes on short lateral branches and forming loose and patchy colonies on calcareous ground.…”

Morphological characters and SNP markers suggest hybridization and introgression in sympatric populations of the pleurocarpous mosses Homalothecium lutescens and H. sericeum

“…H o m a l o t h e c i u m s e r i c e u m a n d H . l u t e s c e n s (Brachytheciaceae) are perennial dioicous pleurocarpous bryophytes commonly found in Europe, showing a more regional distribution in North America (Nyholm 1965;Hofmann 1998;Huttunen et al 2008;Lieske 2010;Rosengren et al 2016;GBIF 2018a, b). The species occur in distinct habitats with a comparatively high pH (median and range of pH is 6.5 and 5.9-7.6 for H. sericeum versus 7.1 and 6.2-7.9 for H. lutescens; Tyler and Olsson 2016); H. sericeum grows epiphytically on trunks of broad-leaved trees or on calcareous stone or mortar in forests or parks, whereas H. lutescens grows on calcareous soil, usually in open habitats such as pastures.…”

“…Spore dispersal by wind in bryophytes is strongly leptokurtic (Miles and Longton 1992), which means that many spores may actually land directly on the sporulating maternal gametophyte. Rosengren et al (2016) inferred that repeated back-crossing by son-mother fertilization occurs from time to time in H. lutescens. For statistical reasons, each back-crossing event will mean that half of the paternal genetic variation is lost.…”

Maintenance of genetic and morphological identity in two siblingSyrrhopodonspecies (Calymperaceae, Bryopsida) despite extensive introgression

“…Specimens of S. annotinus and S. simmondsii clustered within a clade in phylogenetic analyses based on a combination of five chloroplast and one nuclear region (Pereira et al, ), but the lack of molecular variation within that clade called for the use of more variable markers. Single nucleotide polymorphisms (SNPs) have been successfully employed in fine‐scale population genetics of mosses (Rosengren et al, , ) and we, therefore, built an SNP library from a modified genotyping by sequencing (GBS) protocol to test the morphological species concept of S. annotinus and S. simmondsii, address the following questions and test the following hypotheses: to what extent are the spatial genetic and morphological structures of the two species correlated? If S. annotinus and S. simmondsii are reproductively isolated, we expect that reproductive barriers are stronger than isolation‐by‐distance effects, and hence, that there is no relationship between the genetic and morphological similarity of pairs of interspecific individuals and the geographic distance separating them (H1).…”