HYBRIDIZATION AND SPECIATION IN FOSSORIAL MOLE RATS.

Abstract

Hybrid zones between parapatric chromosomal species are of prime evolutionary significance as natural testing grounds for assessing the degree of reproductive isolation between parental species. Major problems of evolutionary dynamics and of species formation may be resolved in hybrid zones. These involve, among others, the stage of speciation, degree of genetic and ecological divergence of the derivatives of speciation, and the types and modes of perfection of isolating mechanisms. This study investigated hybrid zones between chromosome forms of the fossorial mole rat Spalax ehrenbergi as a dynamic model of speciation (Nevo, 1973). Fossorial mole rats of the S. ehrenbergi superspecies complex in Israel involve four main chromosome forms with 2n = 52, 54, 58 and 60. The four karyoptic forms inhabit extensive regions which are distributed clinally and parapatrically along a north-south ecological gradient of increasing aridity (Wahrman et al., 1969). Their distribution is correlated with the humidity index and the climatic regimes as defined by Thornthwaite (1948). Karyotype 2n = 52 is found in the Upper Galilee Mountains in a humid to subhumid climate (55-65 days of rain and 650-1000 mm rainfall annually). Karyotype 2n = 54 extends over Mount Hermon and the Golan Heights which is a subhumid to humid region (4565 days of rain and 500-1400 mm rainfall annually) with lower mean annual temperatures than the previous region. Karyotype 2n = 58 ranges in the Lower Galilee Mountains and Central Israel in a subhumid to semiarid climatic regime (44-55 days of rain and 500-650 mm rainfall annually). Finally, karyotype 2n = 60 is found in the Samaria and Judea Mountains extending toward the southern and eastern deserts of Israel, a semiarid to arid region with some subhumid mountainous enclaves (15-45 days of rain and 100-500 mm rainfall annually). Only rare natural hybrids between karyotypes have been previously reported along the contact zones. The four karyotypes have been considered sibling species on distributional as well as ethological evidence (Nevo, 1969). Further, their extremely close genic relationships (Nevo and Shaw, 1972; Nevo and Sarich, 1975) suggest that they represent very recent species. The present study is a test of their stage of speciation under natural conditions by exploring actual and potential hybrid zones along contacts of contiguous karyotypes. Our present observations of very narrow hybrid zones and lower than expected proportions of hybrids in the narrowest zones suggest an increasing effectiveness of isolating mechanisms between the diverging karyotypes and progressive terminalization of species formation.