Deep mitochondrial introgression and hybridization among ecologically divergent vole species

Abstract

The completion of speciation is typically difficult to ascertain in rapidly diverging taxa but the amount of hybridization and gene flow in sympatry or parapatry contains important information about the level of reproductive isolation achieved. Here, we examined the progress in speciation between the Mediterranean (Microtus duodecimcostatus) and the Lusitanian pine vole (M. lusitanicus), which are part of the most rapid radiation of species known in mammals. These two Iberian pine voles are classified as separate species because of differences in morphology and ecology, but relatively many ambiguous individuals can be found in sympatric conditions. Our phylogenetic analyses of rangewide data from the mitochondrial cytochrome b gene (mtDNA) demonstrated high levels of diversity and a basal separation in two parapatric lineages. However, mtDNA affiliation was at odds with morphological classification or geographical distribution of the taxa. In contrast, statistical analyses of microsatellites (nucDNA) showed two clear genetic clusters in allopatry and sympatry generally matching morphological classification. This cytonuclear discordance over a large geographic area suggests historical introgression of mtDNA from M. duodecimcostatus to M. lusitanicus. There was statistical evidence for at least two recent hybrids in the sympatry zone but gene flow is apparently low given clear-cut differences in nucDNA. Our results indicate a relatively advanced speciation process in these Iberian pine voles without fully established reproductive isolation. This situation enables use of combined population genomic and experimental approaches for the separation of patterns and mechanisms in the ongoing explosive diversification of these and other Arvicoline rodents in the future.

p53 gene discriminates two ecologically divergent sister species of pine voles

“…lusitanicus and M. duodecimcostatus specimens were collected from 21 different sampling sites, throughout the species distribution range in the Iberian Peninsula (Supplementary Table S1). Thirty-one of these individuals had also been studied for cytb sequence diversity in a previous study (Bastos-Silveira et al, 2012). Ten samples from M. duodecimcostatus (from sampling sites 21, 22 and 23) came from the Museo Nacional de Ciencias Naturales, Spain, as well as one sample from M. cabrerae that was used for sequence comparisons within Microtus.…”

“…The two existing population-level studies on the genetic diversity of M. lusitanicus and M. duodecimcostatus in the Iberian Peninsula have found evidence for a past introgressive hybridization from M. duodecimcustatus to M. lusitanicus of the mitochondrial cytochrome b (cytb) gene (Bastos-Silveira et al, 2012), and possibly of the nuclear gene IRBP (interphotoreceptor retinoid-binding protein) given the impossibility to discriminate individuals from the two species when using this marker (Barbosa et al, 2013). Here we performed a population-genetic analysis involving 96 individuals (52 M. lusitanicus and 44 M. duodecimcostatus) sampled in the Iberian Peninsula, and the p53 DNA sequence encoding part of the DNAbinding region of P53, from the 3′ end of exon 5 to the 5′ extreme of exon 7, which includes residue 174.…”

“…Here we performed a population-genetic analysis involving 96 individuals (52 M. lusitanicus and 44 M. duodecimcostatus) sampled in the Iberian Peninsula, and the p53 DNA sequence encoding part of the DNAbinding region of P53, from the 3′ end of exon 5 to the 5′ extreme of exon 7, which includes residue 174. We used specimens captured both in allopatric and sympatric areas where introgressed animals have been observed (Jaarola et al, 2004;Tougard et al, 2008;Bastos-Silveira et al, 2012;Barbosa et al, 2013;Rodriguez-Prieto et al, 2014), to ultimately evaluate whether genes involved in the differentiation of species separated recently carry signature changes reflecting speciation more accurately than neutral genes (Ting et al, 2000). Variability of the obtained p53-coding sequences of M. lusitanicus and M. duodecimcostatus was further compared with the published sequences from other Microtus species, rodents and mammals.…”

Sex-specific clines support incipient speciation in a common European mammal

“…This is likely a consequence of Haldane's rule in place in the hybrid zone, which may-among other factors-prevent the mouse Y-chromosome cline width to increase (Britton-Davidian et al, 2005; but see Turner et al, 2012). The rapidly speciating Microtus genus harbours several other examples of restricted hybridization and evidence for selection acting prevalently on males-typically associated with morphological and karyotypic differences between the involved sibling species or subspecies (Meier et al, 1996;Jaarola et al, 1997;Bulatova et al, 2010;Bastos-Silveira et al, 2012;Beysard et al, 2012). Among the cryptic evolutionary lineages in M. arvalis however, partial reproductive isolation appears to be additionally acting in an unusual unidirectional manner.…”

“…Despite very little morphological differentiation, karyotypic variation and differentiation into deep genetic lineages is relatively common within recognized species, suggesting the presence of cryptic species or ongoing speciation processes (Jaarola et al, 2004;Heckel et al, 2005;Hellborg et al, 2005;Bastos-Silveira et al, 2012). In the common vole (Microtus arvalis), intraspecific divergence has resulted in four main, phenotypically cryptic evolutionary lineages in Europe (Western, Central, Italian and Eastern lineages; Heckel et al, 2005).…”