High diversity of centric fusions with monobrachial homology in an area of chromosomal polymorphism of Mus musculus domesticus

Abstract

One of the simplest models of chromosomal speciation is speciation by monobrachial centric fusion. This model is based on the assumption that a sterility barrier can develop between populations, in which fixed centric fusions show monobrachial homology, i.e. share only one chromosome arm. However, studies aimed at delineating intermediate stages of transition to reproductive isolation are lacking. In this paper, we describe a new area of chromosomal polymorphism in the house mouse, Mus musculus domesticus Schwarz and Schwarx, 1943, in Sicily (Italy). We trapped 79 mice at eighteen localities in an area of approximately 500 Km 2 surrounding the largest active European volcano, Mount Etna. Combining G-banding and chromosome painting we identified twelve different Robertsonian (Rb) metacentrics. Considering the high number of Rb fusions, some of them shared with other documented areas, the presently studied area of chromosomal polymorphism is very likely to represent a mixture of allochthonous and autochthonous Rb fusions. The Rb(9.16) is the most widespread metacentric (overall frequency 0.80). Two Rb metacentrics, Rb(4.10) and Rb(5.6), have similar overall frequency, 0.29 and 0.37, respectively, and are narrowly co-distributed in ten populations. Nine fusions – Rb(2.13), Rb(1.3), Rb(12.17), Rb(8.17), Rb(2.14), Rb(10.14), Rb(11.17), Rb(3.15), and Rb(11.14) – show a low frequency (0.04–0.01) and mostly non-overlapping localization, but each of them shares monobrachial homology with at least one other metacentric. The overall geographical distribution of different Rb fusions seems to match an early stage of race formation. The eventual role of the presently studied hybrid zone in the context of chromosomal speciation by monobrachial centric fusions is discussed. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 722–731. ADDITIONAL KEYWORDS: chromosomal speciation – chromosome races – Mus musculus domesticus – population cytogenetics – Robertsonian metacentrics – structural heterozygotes.