Phylogenetic analysis of the distribution of chromosomal races of Mus musculus domesticus Rutty in Europe

Abstract

Robertsonian (Rb) translocation is a common chromosomal rearrangement in the house mouse. In free-living populations, 79 fusions with different combinations of chromosomes 1 to 18 have been found in some 45 populations. An updated list of these fusions is presented and analysed in order to reveal the possible processes by which the fusions spread within or among populations. A widespread hypothesis is that when two populations share the same fusion, it can be assumed that they have a common ancestor. This can serve as the basis for the use of the cladistic methods. While I present such an analysis on the updated list of Rbs, I also point to the problems associated with it in this case because many fusions have multiple origins and exchanges of Rbs between populations are frequent.I have tried to use a different approach, based on a critical and quantitative evaluation of the hypothesis of common ancestry. Assuming that the 153 possible fusions have an equal probability of occurrence, I give the formula to compute the probability that populations share a given number of fusions by chance alone. Only when this probability is lower than a chosen level (say 5%) can the populations be inferred to have a non-independent origin (i.e. they have a common ancestor or they have exchanged chromosomes by introgression). This probability measure is then used as a distance estimate to show the relationship between all the R b populations. This analysis suggests that although some Rbs must have occurred more than once, most of the populations have nonindependent origins. Almost all the populations from northern Africa to Belgium and Germany appear to have close karyotopic relationships and form a major group. Clearly independent R b populations are mainly found in the periphery of this major group, for example in Scotland, Denmark and Spain. 'Chromosomal' flow between R b populations appears to be a very important process.

Segregation and fertility in Mus musculus domesticus (wild mice) heterozygous for the Rb(4.12) translocation

“…An experimental introduction of Rb mice in a karyotypically normal population has shown that Rb chromosomes can spread very rapidly in a population even though heterozygotes for three Rb metacentrics must have occurred (Berry et a!., 1990). According to his phylogenetic study of Rb populations in Europe, Bauchau (1990) suggests that most of the Robertsonian populations have non-independent origins and that chromosomal flow between these populations is an important process, contrary to what has been understood until now (see also Corti et a!., 1986;Winking eta! 1988).…”

“…However, these fusions are widely distributed among natural populations of Mus musculus domesticus in Western Europe and North Africa (Winking et a!., 1988;Bauchau, 1990). We are thus confronted with a paradox.…”

“…This fusion is also widely spread in many natural populations in Europe, from Belgium to Northern Italy. A phylogenetic analysis (Bauchau, 1990) suggests that this fusion has appeared only once and subsequently spread over a very large area, possibly because of selective forces (see also Corti et a!., 1986;Winking eta!., 1988).…”

Further studies of a staggered hybrid zone in Mus musculus domesticus (the house mouse)

“…Looking at the overall distribution of metacentric races in M. m. domesticus, there is a concentration of such races in high altitude and coastal regions within Europe (Bauchau, 1990;Searle er at., 1990). One would expect particularly small populations in such areas, which may promote fixation of new chromosomal rearrangements (Lande, 1979).…”

“…However, deviation from this standard complement has been documented in various parts of the range of the west European subspecies Mus musculus domesticus (Bauchau, 1990). Here we consider the substantial chromosomal variation in the extreme northern part of mainland Scotland, first discovered by Adolph & Klein (1981) and Brooker (1982).…”