Microgeographical distribution of two chromosomal races of house mice in Tunisia: pattern and origin of habitat partitioning

Abstract

Two chromosomal races of the house mouse occur in Tunisia, a standard morph (40St) found all over the country, and a derived morph (22Rb) occurring only in central Tunisia. In this region, habitat partitioning between the two morphs was investigated by a microgeographical analysis of their distribution, assessing habitat characteristics and demographic parameters. Results showed that the 22Rb mice always occurred in the oldest sections of towns (medinas), often extending to more recent surrounding neighbourhoods where the 40St morph was most abundant. The latter was never trapped within the medinas. The transition between the two morphs was located within cities in the more recent areas, the hybrid zone being estimated at less than 0.5 km in width by a clinal analysis of chromosomal data. Although di¡erences between habitats exist, almost no demographic di¡erences were found between populations of the two morphs when they occurred in the same or in di¡erent habitats. Two hypotheses are discussed to account for the origin of habitat partitioning. The ¢rst relies on competitive exclusion of the 40St mice from the medinas by the derived 22Rb mice; the second is based on stochastic processes related to historical evolution of Tunisian urban communities.

Spatio-temporal variation in the structure of a chromosomal polymorphism zone in the house mouse

“…The arrival of allochthonous mice in the centre of a metacentric zone has also been reported in Rb populations from Italy (Franchini et al, 2007;Castiglia and Capanna, 1999) and Madeira (Nunes et al, 2005). In those cases, as may also occur in the Rb zone of Barcelona, the action of humans on the natural landscape has influenced the dynamics of commensal mice (Chatti et al, 1999;Castiglia and Capanna, 1999). Passive transport via vehicles, especially in farming areas, may have promoted introgression events independently of the common mouse dispersion (Cucchi et al, 2005;Franchini et al, 2007).…”

Contact zones between chromosomal races of Mus musculus domesticus. 3. Molecular and chromosomal evidence of restricted gene flow between the CD race (2n = 22) and the ACR race (2n = 24)

“…Such a level of genetic isolation has only been observed between the POS and UV races in Valtellina (northern Italy) (Capanna and Corti, 1982) and in some contact Heredity areas in the hybrid zone between the Monastir race and the standard race in Tunisia (Said and Britton-Davidian, 1991;Chatti et al, 1999). However, no detailed molecular studies are available for these zones.…”

“…However, if the differences between races involve many independent metacentrics the fertility of structural heterozygotes is more severely impaired (Redi and Capanna, 1988;Said et al, 1993;Castiglia and Capanna, 2000). In these extreme cases, both gene flow and the number of hybrid individuals is expected to be low, as in the Tunisian hybrid zone between the Monastir race 2n = 22 and the standard one (Chatti et al, 1999).…”

“…Many studies have focused on the contact zones between Robertsonian races and the standard one (see Searle, 1993 for a review; Castiglia and Capanna, 1999a;Gundü z et al, 2001a). All these studies have found hybrids showing trivalents at diakinesis, with hybrid zone widths from 0.5 km (Chatti et al, 1999) to 30 km . Fertility studies show that gene flow should occur freely between races differing in only a few independent metacentrics: heterozygotes for 1-3 metacentrics have normal levels of fertility (Winking et al, 1988;Wallace et al, 1992;Castiglia and Capanna, 2000).…”

Evidence for a mitochondrial lineage originating from the Arabian peninsula in the Madagascar house mouse (Mus musculus)

“…The female-biased sex-ratio observed here is common in commensal populations and high-density populations (Auffray, 1988), while outdoor populations usually have an excess of males (Ritte et al, 1992;Chatti et al, 1999), as do low-density populations (Sage, 1981).…”