Abstract
Among plants, gender dimorphism occurs in about 10% of all angiosperms and more than 50% of all moss taxa, with dwarf males (DM) found exclusively in some unisexual mosses. In this study, we explore the role of male dwarfism as a reproductive strategy in the widespread acrocarpous moss Dicranum scoparium, which has facultative male dwarfism, having both dwarf males (DMs) and normal-sized males (NMs). We retrieved 119 SNP markers from transcriptomes which were used to genotype 403 samples from 11 sites at seven localities in southern Sweden. Our aims were to compare the genetic variability and genetic structure of sexually reproducing populations at different geographic levels (cushion, site, and locality) and compare in particular the relative contribution of females, dwarf males and normal-sized males to the observed genetic diversity. The numbers of DMs differed strongly between sites, but when present, they usually outnumbered both females and NMs. Low genetic differentiation was found at locality level. Genetic differentiation was strongest between cushions for females and NMs and within cushions for DMs indicating small scale structuring and sometimes inbreeding. NMs were more clonal than either DMs or females. Genetic diversity was similar between females and DMs, but lower for NMs. Two haplotypes were shared between females and DMs and one haplotype was shared between a DM and a NM. In conclusion, our results show that DMs and NMs play different roles in reproduction, inbreeding may occur at cushion level, but gene flow is high enough to prevent substantial genetic drift.
Highlights
Unisexuality is usually associated with a certain degree of sexual dimorphism (Badyaev, 2002; Poissant et al, 2010)
In this study we focus on the widespread species D. scoparium sampled from seven localities, using 90 single-nucleotide polymorphic markers (SNPs)
Clonality occurred in all three categories, yet at different degrees: level of clonality was high among normal-sized males (NMs) whereas it was much less so in females and dwarf males (DMs)
Summary
Unisexuality is usually associated with a certain degree of sexual dimorphism (Badyaev, 2002; Poissant et al, 2010). A form of extreme size dimorphism where male measure at most half of the size of females, is most common in marine taxa (Fairbairn et al, 2007) and in small organisms, such as invertebrates, fishes, algae, and bryophytes It might occur when (4) the probability to encounter a sexual partner is low and the need to keep sexual partners nearby is high, as for example in the bone-eating marine worm Osedax or anglerfishes (Vollrath, 1998; Pietsch, 2005; Vrijenhoek et al, 2008; Rouse et al, 2015): in these cases males evolved into rudimentary creatures, which are permanently attached to a normal-sized female. Called nannandry, has evolved several times independently in unrelated moss families (Vanderpoorten and Goffinet, 2009; Hedenäs and Bisang, 2011) and can either be obligate or facultative, i.e., males growing only as dwarfs or occurring as normal-sized males as well
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