Incomplete Sterility of Chromosomal Hybrids: Implications for Karyotype Evolution and Homoploid Hybrid Speciation

Abstract

Heterozygotes for major chromosomal rearrangements such as fusions and fissions are expected to display a high level of sterility due to problems during meiosis. However, some species, especially plants and animals with holocentric chromosomes, are known to tolerate chromosomal heterozygosity even for multiple rearrangements. Here, we studied male meiotic chromosome behavior in four hybrid generations (F1-F4) between two chromosomal races of the Wood White butterfly Leptidea sinapis differentiated by at least 24 chromosomal fusions/fissions. Previous work showed that these hybrids were fertile, although their fertility was reduced as compared to crosses within chromosomal races. We demonstrate that (i) F1 hybrids are highly heterozygous with nearly all chromosomes participating in the formation of trivalents at the first meiotic division, and (ii) that from F1 to F4 the number of trivalents decreases and the number of bivalents increases. We argue that the observed process of chromosome sorting would, if continued, result in a new homozygous chromosomal race, i.e., in a new karyotype with intermediate chromosome number and, possibly, in a new incipient homoploid hybrid species. We also discuss the segregational model of karyotype evolution and the chromosomal model of homoploid hybrid speciation.

Genomic, karyological and morphological changes of South American garlics (Ipheion) provide insights into mechanisms of speciation in the Pampean region

“…In long-lived plants, even low levels of fertility of interspecific hybridsincluding interploidy-can affect rates of gene flow between lineages (see, e.g., Pinheiro et al, 2010). In the case of heterozygotic taxa, for major chromosomal rearrangements, such as fusions and fissions, a few plant studies (see below) and many more examples from animals point to a high level of hybrid sterility due to problems during meiosis (Lukhtanov et al, 2020). Thus, in Ipheion, species divergence was probably fostered also by a high degree of hybrid sterility reinforcing reproductive isolation mechanisms.…”

The effects of GC-biased gene conversion on patterns of genetic diversity among and across butterfly genomes

“…Neither genetic diversity, (π ; p ≈ 0.13, adjusted R 2 ≈ 0.45), nor diploid chromosome number ( p ≈ 0.35, R 2 ≈ 0.05), significantly predicted variation in B among species in phylogenetically independent contrasts (Figure 2C, D). Since Spanish L. sinapis likely experienced massive chromosomal fission events recently (Lukhtanov, et al 2011; Talla, Johansson, et al 2019; Lukhtanov, et al 2020), it is possible that B is below its equilibrium value in this population. Excluding Spa-sin yielded a marginally significant positive relationship between chromosome number and the intensity of gBGC ( p ≈ 0.07, R 2 ≈ 0.79).…”

Karyotype and DNA barcode of Polyommatus (Agrodiaetus) cyaneus (Staudinger, 1899) from its type locality: implication for taxonomic and evolutionary research in Polyommatus blue butterflies (Lepidoptera, Lycaenidae)

“…Cells in which the chromosomes were not located on the same plane, as well as cells with overlapping or touching chromosomes and/or bivalents, were rejected and not used for analysis. In some cases, diploid chromosome number (2n) was counted in atypical male meiosis which represent a kind of asynaptic meiosis (Lorković 1990;Lukhtanov and Dantchenko 2017;Lukhtanov et al 2020b).…”