Chromosomal speciation and adaptive radiation of mole rats in Asia Minor correlated with increased ecological stress.

Abstract

The evolutionary forces causing chromosomal speciation and adaptation are still enigmatic. Here we tested the Israel evolutionary model of positive association of diploid chromosome number (2n) and genetic diversity with aridity stress in subterranean mole rats, on a 30-times-larger scale in Asia Minor. We analyzed both karyotype and allozyme diversity across Turkey, based on 37 allozymic loci in 20 localities of the Spalax leucodon and 4 localities of the Spalax ehrenbergi superspecies. We found extensive chromosomal speciation in S. leucodon (2n = 38, 40, 50, 54, 60, and 62) and in S. ehrenbergi (2n = 52, 56, and 58), presumably representing from 14 to > 20 additional biological species. Genetic diversity indices were low, but, like the chromosome number (2n), positively correlated with aridity stress, increasing centripetally from the periphery toward geologically young, arid, and climatically unpredictable central Anatolia. Nei's genetic distance D across all populations averaged 0.174 (range 0.002-0.422), supporting, combined with 2n and ecogeography, the biological species status of most tested populations. Chromosome evolution is the basis of speciation and adaptation in Spalax; it provides both postmating reproductive isolation, as well as higher levels of recombination with increased 2n. A mathematical model shows that a Robertsonian fission of a single metacentric considerably increases haplotype diversity. This haplotype diversity may contribute to population adaptation to climatic stress and ecological unpredictability in space and time. The increase in diversity corroborates the nichewidth genetic-variation hypothesis.