Restricted Gene Flow at Specific Parts of the Shrew Genome in Chromosomal Hybrid Zones

Abstract

Abstract. The species and races of the shrews of the Sorex araneus group exhibit a broad range of chromosomal polymorphisms. European taxa of this group are parapatric and form contact or hybrid zones that span an extraordinary variety of situations, ranging from absolute genetic isolation to almost free gene flow. This variety seems to depend for a large part on the chromosome composition of populations, which are primarily differentiated by various Robertsonian fusions of a subset of acrocentric chromosomes. Previous studies suggested that chromosomal rearrangements play a causative role in the speciation process. In such models, gene flow should be more restricted for markers on chromosomes involved in rearrangements than on chromosomes common in both parent species. In the present study, we address the possibility of such differential gene flow in the context of two genetically very similar but karyotypically different hybrid zones between species of the S. araneus group using microsatellite loci mapped to the chromosome arm level. Interspecific genetic structure across rearranged chromosomes was in general larger than across common chromosomes. However, the difference between the two classes of chromosomes was only significant in the hybrid zone where the complexity of hybrids is expected to be larger. These differences did not distinguish populations within species. Therefore, the rearranged chromosomes appear to affect the reproductive barrier between karyotypic species, although the strength of this effect depends on the complexity of the hybrids produced.

Chromosomal rearrangements and gene flow over time in an inter-specific hybrid zone of the Sorex araneus group

“…However, the inter-specific difference between the two classes of chromosomes was significant only in the centre of the hybrid zone where the two species lived in sympatry and where there is no evident physical barrier to gene flow. This difference was not significant over a larger geographic range (Basset et al, 2006a) and such results remain difficult to interpret.…”

“…Some individuals have already been genotyped at some loci as part of previous study (Brü nner et al, 2002b;Basset et al, 2006a;Yannic et al, 2008a) and were not regenotyped here (see Table 1). …”

“…When comparing the karyotype of the taxa involved in this hybrid zone, it is then possible to define (i) one group of chromosomes similarly arranged as common acrocentrics or metacentrics, and (ii) one group of chromosomes rearranged in different acrocentrics or metacentrics according to the species (Figure 1). Recently, Basset et al (2006a) used microsatellite loci mapped to individual chromosome arms to test the possibility of an impact of chromosomal rearrangement on gene flow between S. araneus and S. antinorii in the Haslital hybrid zone. These authors showed that inter-specific genetic structure across rearranged chromosomes was indeed larger than that across similarly arranged chromosome (that is, 'common' Figure 1 Common and rearranged chromosomes in the parent taxa and their combination in F 1 hybrids.…”

“…In this study, we assess the dynamics of the Haslital hybrid zone over time, by comparing data sampled between 1992 and 1995 (Basset et al, 2006a), with data collected 10 years later (that is, between 2004 and 2005).…”

“…Our main aim was to detect a hybrid zone movement over time, by studying hybrid zone genetic structure using microsatellite markers, and to compare the genetic structure between common and rearranged chromosomes in the two periods using loci mapped at the chromosome arm level to confirm the pattern observed by Basset et al (2006a). To our knowledge, this is the first study that addresses the question of differential gene flow in natural populations, by comparing the level of introgression over time on different parts of the genome.…”