Abstract
BackgroundSpecies generally have a fixed number of chromosomes in the cell nuclei while between-species differences are common and often pronounced. These differences could have evolved through multiple speciation events, each involving the fixation of a single chromosomal rearrangement. Alternatively, marked changes in the karyotype may be the consequence of within-species accumulation of multiple chromosomal fissions/fusions, resulting in highly polymorphic systems with the subsequent extinction of intermediate karyomorphs. Although this mechanism of chromosome number evolution is possible in theory, it has not been well documented.ResultsWe present the discovery of exceptional intraspecific variability in the karyotype of the widespread Eurasian butterfly Leptidea sinapis. We show that within this species the diploid chromosome number gradually decreases from 2n = 106 in Spain to 2n = 56 in eastern Kazakhstan, resulting in a 6000 km-wide cline that originated recently (8,500 to 31,000 years ago). Remarkably, intrapopulational chromosome number polymorphism exists, the chromosome number range overlaps between some populations separated by hundreds of kilometers, and chromosomal heterozygotes are abundant. We demonstrate that this karyotypic variability is intraspecific because in L. sinapis a broad geographical distribution is coupled with a homogenous morphological and genetic structure.ConclusionsThe discovered system represents the first clearly documented case of explosive chromosome number evolution through intraspecific and intrapopulation accumulation of multiple chromosomal changes. Leptidea sinapis may be used as a model system for studying speciation by means of chromosomally-based suppressed recombination mechanisms, as well as clinal speciation, a process that is theoretically possible but difficult to document. The discovered cline seems to represent a narrow time-window of the very first steps of species formation linked to multiple chromosomal changes that have occurred explosively. This case offers a rare opportunity to study this process before drift, dispersal, selection, extinction and speciation erase the traces of microevolutionary events and just leave the final picture of a pronounced interspecific chromosomal difference.
Highlights
Species generally have a fixed number of chromosomes in the cell nuclei while between-species differences are common and often pronounced
In this study we describe a chromosomal cline in the Wood White butterfly, Leptidea sinapis (Insecta, Lepidoptera, Pieridae) that provides strong evidence for rapid and extensive within-species chromosome number evolution through accumulation of multiple chromosomal changes
As all karyotype data for L. sinapis were published before this date, it is unclear whether reported chromosome numbers reflect inter- or intraspecific variability
Summary
Species generally have a fixed number of chromosomes in the cell nuclei while between-species differences are common and often pronounced. Marked changes in the karyotype may be the consequence of within-species accumulation of multiple chromosomal fissions/fusions, resulting in highly polymorphic systems with the subsequent extinction of intermediate karyomorphs. This mechanism of chromosome number evolution is possible in theory, it has not been well documented. Main karyotypic features of organisms, such as the number of chromosomes, are usually stable within species [1,2] This stability is in good correspondence with the fact that new chromosomal rearrangements usually originate as heterozygotes and are often - not always - associated with heterozygote disadvantage (=negative heterosis; =underdominance).
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