Abstract
BackgroundChromosomal rearrangements have the potential to limit the rate and pattern of gene flow within and between species and thus play a direct role in promoting and maintaining speciation. Wood white butterflies of the genus Leptidea are excellent models to study the role of chromosome rearrangements in speciation because they show karyotype variability not only among but also within species. In this work, we investigated genome architecture of three cryptic Leptidea species (L. juvernica, L. sinapis and L. reali) by standard and molecular cytogenetic techniques in order to reveal causes of the karyotype variability.ResultsChromosome numbers ranged from 2n = 85 to 91 in L. juvernica and 2n = 69 to 73 in L. sinapis (both from Czech populations) to 2n = 51 to 55 in L. reali (Spanish population). We observed significant differences in chromosome numbers and localization of cytogenetic markers (rDNA and H3 histone genes) within the offspring of individual females. Using FISH with the (TTAGG)n telomeric probe we also documented the presence of multiple chromosome fusions and/or fissions and other complex rearrangements. Thus, the intraspecific karyotype variability is likely due to irregular chromosome segregation of multivalent meiotic configurations. The analysis of female meiotic chromosomes by GISH and CGH revealed multiple sex chromosomes: W1W2W3Z1Z2Z3Z4 in L. juvernica, W1W2W3Z1Z2Z3 in L. sinapis and W1W2W3W4Z1Z2Z3Z4 in L. reali.ConclusionsOur results suggest a dynamic karyotype evolution and point to the role of chromosomal rearrangements in the speciation of Leptidea butterflies. Moreover, our study revealed a curious sex determination system with 3–4 W and 3–4 Z chromosomes, which is unique in the Lepidoptera and which could also have played a role in the speciation process of the three Leptidea species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0375-4) contains supplementary material, which is available to authorized users.
Highlights
Chromosomal rearrangements have the potential to limit the rate and pattern of gene flow within and between species and play a direct role in promoting and maintaining speciation
Phylogenetic analyses based on two DNA markers, the mitochondrial cytochrome c oxidase subunit 1 (COI) gene and the nuclear internal transcribed spacer 2 (ITS2) sequence, revealed three supported major clades corresponding to L. juvernica, L. sinapis and L. reali (Fig. 1 and Additional file 1: Figure S1)
Mitotic complements of L. juvernica and L. sinapis displayed a higher variability in chromosome size, having mostly middle- or small-sized chromosomes (L. juvernica, Fig. 2a) or a mixture of large- and small-sized chromosomes (L. sinapis, Fig. 3c, d, f), while in L. reali we observed larger chromosomes of a similar size (Fig. 2b)
Summary
Chromosomal rearrangements have the potential to limit the rate and pattern of gene flow within and between species and play a direct role in promoting and maintaining speciation. Wood white butterflies of the genus Leptidea are excellent models to study the role of chromosome rearrangements in speciation because they show karyotype variability among and within species. Speciation, i.e. the origin of new species, is a complex evolutionary process which leads to the formation of barriers preventing gene flow between emerging species. The Heliconius butterflies have been the subject of a high number of evolutionary studies showing that various wing patterns, resulting from predator-induced selection through Müllerian mimicry, lead to divergence and speciation. Lepidoptera include models for research of sex pheromone communication and its role as a pre-zygotic barrier [11, 12]. The butterfly subgenus Agrodiaetus is one of the few taxa where reinforcement of pre-zygotic isolation has been demonstrated [15]
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