Sympatric moss populations can form true hybrid zones

Hybridization between species has not been considered an important factor in the evolution of mosses. The moss life cycle has led many researchers to believe that the plants have very limited evolutionary potential and that hybrids tend to be nonviable. Complicating the study of moss hybrids is the fact that a lack of distinguishing morphological characters means it must be done using genetic markers. Whether hybridization holds evolutionary significance for mosses depends on how much genetic mixing takes place, and the fitness and reproductive capacity of hybrid individuals.

In a recent article published in Annals of Botany, W. Sawangproh and colleagues used single nucleotide polymorphism (SNP) markers to determine the extent and viability of genetic mixing between two mosses, Homalothecium lutescens and H. sericeum, growing in southern Sweden. The researchers analyzed 449 moss samples from seven mixed (sympatric) and five pure (allopatric) populations representing three generations – haploid maternal gametophytes, diploid sporophytes, and haploid sporelings. The markers were species-specific and allowed the identification of mildly versus strongly mixed individuals.

The researchers found that though the majority of mosses tested were pure species, a significant minority were hybrids. Mildly admixed hybrids made up 17% of the mosses tested, while strongly admixed individuals made up a mere 3.8%. All three generations contained admixed samples, and gene movement occurred in both parental directions. The transfer of mixed genomes from one generation to the next means that the mosses can form true hybrid zones, where introduced genes can potentially be exposed to selection and contribute to fitness and possible speciation.

“Earlier studies of bryophyte populations with admixed individuals have not been able to establish whether the admixed individuals are fertile and the populations represent true introgression zones,” the authors write. “It is evident from our study that admixed gametophytes of Homalothecium can produce sporophytes and subsequently viable sporelings that show detectable signals for most of the SNP markers, indicating that they have been successfully transmitted from sporophyte to progeny.” The authors note that complete fusion of species may not be possible because strongly admixed samples, which represented a small minority of those tested, are “subject to hybrid depression dependent on break up of adaptive gene complexes that are necessary for survival in the typical habitats of either species.”