Extreme Karyotypic Variation in a Mus musculus domesticus Hybrid Zone: The Tobacco Mouse Story Revisited

Abstract

The Robertsonian fusion is a common chromosomal mutation among mammal species and is especially prevalent in the West European house mouse, Mus musculus domesticus. More than 40 races of the house mouse exist in Europe, including the famous "tobacco mouse" (Poschiavo race) of Val Poschiavo, Switzerland. Documented here is the discovery of an extreme case of karyotypic variation in the neighboring Upper Valtellina, Italy. In a 20-km stretch of the valley, 32 karyotypes were observed, including five chromosomal races and 27 hybrid types. One previously unknown race is reported, the "Mid Valtellina" race, with a diploid number of 2n = 24 and the Robertsonian fusions Rb(1.3), Rb(4.6), Rb(5.15), Rb(7.18), Rb(8.12), Rb(9.14), Rb(11.13), and Rb(16.17). The Poschiavo race (2n = 26), Upper Valtellina race (2n = 24), Lower Valtellina race (2n = 22) and all-acrocentric race (2n = 40) were also present. The races form a patchy distribution, which we term a "mottled hybrid zone." Geographical position, isolation, extinction, recolonization, and selection against hybrids are all believed to be instrumental in the origin and evolution of this complex system. Previous studies of house mice from Upper Valtellina indicated that two of the races in the valley (the Upper Valtellina and Poschiavo races) may have speciated in the village of Migiondo. We discuss the possibility that there may have been a reinforcement event in this village.

Revisiting the Impact of Inversions in Evolution: From Population Genetic Markers to Drivers of Adaptive Shifts and Speciation?

“…Lastly, inversions may promote sympatric or parapatric speciation by creating associations between alleles under divergent natural selection and those that cause assortative mating (Butlin 2005, Hauffe & Searle 1993, Trickett & Butlin 1994). Also, if the inversion (or incompatibilities associated with it) causes some degree of postzygotic isolation, then selection may favor alleles that increase the strength of prezygotic reproductive barriers (i.e., reinforcement), but only where populations are in contact (Butlin 2005, Servedio & Noor 2003).…”

Clinal analysis of a chromosomal hybrid zone in the house mouse

“…Hybrids between the Barcelona race and the standard race are found over a very large area (over 5000 km#) and have a huge variety of different karyotypes (35 recorded) but have never been found to be heterozygous for more than three metacentrics. The Barcelona hybrid zone is not like the narrow zones involving house mice in central and northern Italy and Tunisia, which are characterized by highly unfit hybrids (Hauffe & Searle, 1993 ;Castiglia & Capanna, 1999 ;Saı$ d et al, 1999). It is the occurrence of such highly unfit hybrids which may lead to speciation by the reinforcement process.…”

“…Hybrid zones are potential sites of speciation, particularly by the process of ' reinforcement ' (Noor, 1999), and the analysis of clinal variation within zones can provide much insight into hybrid unfitness (Barton & Gale, 1993). In the house mouse, there have been detailed studies of chromosomal hybrid zones in Belgium (Bauchau et al, 1990 ;Hu$ bner & Koulischer, 1990), Denmark (Fel-Clair et al, 1996), central Italy (Spirito et al, 1980 ;Castiglia & Capanna, 1999), northern Italy (Hauffe & Searle, 1993), northern Scotland (Searle et al, 1993) and Tunisia (Saı$ d et al, 1999). In the case of the north Italian and Tunisian hybrid zones, there is evidence of strong hybrid unfitness (Saı$ d et al, 1993 ;Hauffe & Searle, 1998), and a possibility that this unfitness has promoted reinforcement (Saı$ d et al, 1999 ;Hauffe & Searle, 1992).…”

Staggered Chromosomal Hybrid Zones in the House Mouse: Relevance to Reticulate Evolution and Speciation

“…Within the 20 km section of alpine valley in northern Italy known as ‘Upper Valtellina’ there are two chromosomal races that have not been observed elsewhere: IMVA and IUVA [14,30,31]. Flanking these two races and extending into neighbouring areas are two other races: CHPO and ILVA [31,32].…”

“…Flanking these two races and extending into neighbouring areas are two other races: CHPO and ILVA [31,32]. On grounds of karyotype, phylogeny and distribution [14,31], the IMVA and IUVA races appear to be the products of hybridisation of the CHPO and ILVA races and therefore examples of ‘zonal raciation’. In Table 2 it can be seen that both the IMVA and IUVA races have a combination of chromosomes that could be derived from the CHPO and ILVA races.…”