Abstract
Interspecific hybridization, polyploidization and transitions from sexuality to asexuality considerably affect organismal genomes. Especially the last mentioned process has been assumed to play a significant role in the initiation of chromosomal rearrangements, causing increased rates of karyotype evolution. We used cytogenetic analysis and molecular dating of cladogenetic events to compare the rate of changes of chromosome morphology and karyotype in asexually and sexually reproducing counterparts in European spined loach fish (Cobitis). We studied metaphases of three sexually reproducing species and their diploid and polyploid hybrid clones of different age of origin. The material includes artificial F1 hybrid strains, representatives of lineage originated in Holocene epoch, and also individuals of an oldest known age to date (roughly 0.37 MYA). Thereafter we applied GISH technique as a marker to differentiate parental chromosomal sets in hybrids. Although the sexual species accumulated remarkable chromosomal rearrangements after their speciation, we observed no differences in chromosome numbers and/or morphology among karyotypes of asexual hybrids. These hybrids possess chromosome sets originating from respective parental species with no cytogenetically detectable recombinations, suggesting their integrity even in a long term. The switch to asexual reproduction thus did not provoke any significant acceleration of the rate of chromosomal evolution in Cobitis. Asexual animals described in other case studies reproduce ameiotically, while Cobitis hybrids described here produce eggs likely through modified meiosis. Therefore, our findings indicate that the effect of asexuality on the rate of chromosomal change may be context-dependent rather than universal and related to particular type of asexual reproduction.
Highlights
Most species differ by their karyotype, which are routinely defined by the variation in number, size and morphology of chromosomes
Sexual parental species included in this study are C. elongatoides, C. taenia and C. tanaitica and hereafter we use the acronyms E, T and N for their haploid genomes, respectively
Numbers of chromosomes in particular morphological categories of all hybrids matched those expected by combining the karyotypes of parental species
Summary
Most species differ by their karyotype, which are routinely defined by the variation in number, size and morphology of chromosomes. Differences originating during karyotype evolution play an important role in speciation [1]. Changes in chromosome number have occurred roughly 20 times faster in mammals than in frogs [2] With more than 27.000 taxonomically known species [4], teleost fishes show rather conserved karyotypes with a uniform haploid chromosome number n = 24–25 or close to it [5,6], suggesting a slow rate of karyotype change, mostly by intrachromosomal rearrangements [7]. A number of exceptions exist, especially in lineages of polyploid origin (see [8])
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