Gene transfer across species boundaries in bryophytes: evidence from major life cycle stages in Homalothecium lutescens and H. sericeum

Abstract

Background and Aims The mosses Homalothecium lutescens and H. sericeum are genetically, morphologically and ecologically differentiated; mixed populations sometimes occur. In sympatric populations, intermediate character states among gametophytes and sporophytes have been observed, suggesting hybridization and introgression in such populations. Methods We determined genotypes using bi-allelic co-dominant single nucleotide polymorphism (SNP) markers, specific to either H. lutescens or H. sericeum, to estimate the degree of genetic mixing in 449 moss samples collected from seven sympatric and five allopatric populations on the island of Öland, south Sweden. The samples represented three generations: haploid maternal gametophytes; diploid sporophytes; and haploid sporelings. Key Results Admixture analyses of SNP genotypes identified a majority as pure H. lutescens or H. sericeum, but 76 samples were identified as mildly admixed (17 %) and 17 samples (3.8 %) as strongly admixed. Admixed samples were represented in all three generations in several populations. Hybridization and introgression were bidirectional. Conclusions Our results demonstrate that admixed genomes are transferred between the generations, so that the populations behave as true hybrid zones. Earlier studies of sympatric bryophyte populations with admixed individuals have not been able to show that admixed alleles are transferred beyond the first generation. The presence of true hybrid zones has strong evolutionary implications because genetic material transferred across species boundaries can be directly exposed to selection in the long-lived haploid generation of the bryophyte life cycle, and contribute to local adaptation, long-term survival and speciation.

Evolutionary History of the Marchantia polymorpha Complex

“…Maybe, more so for bryophytes than for many other organisms if multiple hybridization events with transfer of small parts of the genome at a time is a widespread phenomenon (cf. Natcheva and Cronberg, 2007;Sawangproh et al, 2020).…”

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

“…Finally, sporophytes with intermediate capsule inclination were more commonly borne on gametophytes with a maternal origin from H. lutescens, as indicated by the SNP markers. In a subsequent larger study, comparing hybridization across different stages in the life cycle (Sawangproh et al 2020), we did find a small fraction of individuals (3.8%) with strongly admixed genomes suggesting primary hybridization.…”

“…The seta is the stalk supporting the capsule of a moss, and a part of the diploid sporophyte and therefore in principle containing a complete set of chromosomes from each parental gametophyte if hybridization occurs. The reason for not extracting also the capsule was that we planned to sow the spores to test spore germination in a separate study (Sawangproh et al 2020;Sawangproh and Cronberg, in prep). Our initial plan was to also analyse the haplotypes of the maternal individuals, but this was not done due to a temporary problem with extraction failure (see Table 2).…”

“…We knew beforehand that we were close to the detection limit, although we used a method for SNP genotyping that required low amounts of DNA. The reason for extracting setae was that we wanted to sow out the spores in the spore capsules for germination tests (Sawangproh et al 2020;Sawangproh and Cronberg, in prep). It is possible that heterozygotic loci are overrepresented among the failing loci, since each SNP allele only represents half the amount of DNA as compared with the single allele in a homozygotic locus.…”

“…Such individuals may possess more strongly mixed genomes. In additional data sets from the same sympatric populations we include non-sporulating gametophytes, a few of which appear to be more close to primary hybrids, and we also perform germination tests to estimate spore germination frequencies (Sawangproh et al 2020;Sawangproh and Cronberg, in prep).…”