The nature of intraspecific genome size variation in taxonomically complex eyebrights

Abstract

Genome size (GS) is a key trait related to morphology, life history, and evolvability. Although GS is, by definition, affected by presence/absence variants (PAVs), which are ubiquitous in population sequencing studies, GS is often treated as an intrinsic property of a species. Here, we studied intra- and interspecific GS variation in taxonomically complex British eyebrights (Euphrasia). We generated GS data for 192 individuals of diploid and tetraploid Euphrasia and analysed GS variation in relation to ploidy, taxonomy, population affiliation, and geography. We further compared the genomic repeat content of 30 samples. We found considerable genuine intraspecific GS variation, and observed isolation-by-distance for GS in outcrossing diploids. Tetraploid Euphrasia showed contrasting patterns, with GS increasing with latitude in outcrossing Euphrasia arctica, but little GS variation in the highly selfing Euphrasia micrantha. Interspecific differences in GS genomic repeat percentages were small. We show the utility of treating GS as the outcome of polygenic variation. Like other types of genetic variation, such as single nucleotide polymorphisms, GS variation may be increased through hybridisation and population subdivision. In addition to selection on associated traits, GS is predicted to be affected indirectly by selection due to pleiotropy of the underlying PAVs.

Is there hybridization between diploid and tetraploid Euphrasia in a secondary contact zone?

“…Out of 71 hybrid Euphrasia combinations reported in the British flora based on morphology, 13 are reported to be diploid‐tetraploid hybrids (Stace et al, 2015; Figure 1A). Unusually for cross‐ploidy hybrids, these are purported to be diploids, derived from triploid F 1 hybrids (Yeo, 1956), with the putative cross‐ploidy hybrid E. anglica × E. nemorosa confirmed as diploid by flow cytometry (Becher et al, 2021). In this proposed cross‐ploidy hybridization scenario, the subgenome of the tetraploid species homologous to extant diploids can successfully pair and recombine, while the divergent tetraploid subgenome is rapidly lost in meiosis (Figure 1B).…”