MECHANISMS OF FIXATION AND ACCUMULATION OF CENTRIC FUSIONS IN NATURAL POPULATIONS OF MUS MUSCULUS L. I. KARYOLOGICAL ANALYSIS OF A HYBRID ZONE BETWEEN TWO POPULATIONS IN THE CENTRAL APENNINES.

Abstract

Chromosomal mutations causing partial sterility of the heterozygote are decisive in speciation processes in animals of limited vagility (e.g., Orthoptera and small mammals). In this regard the stasipatric speciation model (production of a heterozygous mutant, fixation in a small colony and subsequent spread of the homozygous mutant population) was proposed (see White, 1968, 1973, 1975, 1977), as opposed to the hypothesis of allopatric speciation (e.g., Key, 1974). Rodents offer several suitable characteristics for this type of study: 1) A variety of chromosomal rearrangements are involved: pericentric inversions, as in Spalax (Soldatovic and Savic, 1967; Wahrman et al., 1969; Lay and Nadler, 1972), Thomomys (Patton and Dingman, 1970), Perognathus (Patton, 1969), Peromyscus (Hsu and Arrighi, 1968) and, above all, centric fusions-and to a lesser extent, fissions-in Spalax (Wahrman et al., 1969; Lay and Nadler, 1972), Thomomys (Thaeler, 1968a; Nevo et al., 1974), Perognathus (Patton, 1969), Gerbillus (Wahrman and Gourevitz, 1973), Acomys (Wahrman and Goitein, 1972), Geomys (Selander et al., 1974) and Leggada (Matthey, 1970). 2) Different steps in the speciation process are evident: among different species of the same genus, among sibling species, among incipient species and among interbreeding populations. In Thomomys (Thaeler, 1968a, 1968b; Patton and Dingman, 1970;