Cytological studies of Blattella germanica and Blattella asahinai. I. A possible genetic basis of interspecific divergence

Abstract

The position of the nucleolus organizing region (NOR) in Blattella asahinai differs from that in its close relative, Blattella germanica. The number and position of NORs in hybrid males from reciprocal crosses indicate that the NOR carried by the X chromosome of B. germanica corresponds to a small, paired, terminal segment on the longest chromosome (No. 12) in B. asahinai. The segment is not present in B. germanica; hybrids are heterozygous for the segment. Except for these differences and a relatively smaller size of the B. asahinai X chromosome, gross chromosome morphology is essentially identical in the two species and their hybrids. The size of the X and position of the NORs in hybrid males were as expected according to the direction of the cross. The observations are clearly indicative of a nonreciprocal translocation involving the NOR on the X chromosome of B. germanica and those on an autosome of B. asahinai. The shift involved genes fundamental to protein synthesis, i.e., rDNA (identified by association with the nucleoli). Even minor effects on these genes could bring about significant changes in an organism. It is suggested that the translocation may have been the genetic mechanism of interspecific divergence.Key words: cockroach, evolution, nucleolus, NOR chromosomes.