DICHROPLUS-PRATENSIS AND SOREX-ARANEUS - 2 EXAMPLES OF CHROMOSOMAL HYBRID ZONES IN THE CONTEXT OF SPECIATION

Different chromosomal races of the South American grasshopper Dichropl us pratensis differ substantially in their complement of metacentric c hromosomes. Sometimes the metacentrics in different races show monobra chial homology, presumably due to independent Robertsonian fusions. At least one contact zone between two chromosomal races is known, in whi ch individuals forming meiotic quadri- and quinquevalents are observed . There are several characteristics which make this hybrid zone quite different from that of the common shrew (Sorex araneus) in Britain, wh ich likewise occurs at the contact between metacentric races: 1) all p opulations sampled, whether within or outside the contact area, are po lymorphic for the same arm combinations that differ between the races; 2) the frequencies of the metacentric chromosomes are invariably high even within the contact area; and 3) metacentric frequencies vary rat her abruptly over short distances. A comparison between the chromosoma l hybrid zones of D. pratensis and S. araneus is made in relation to c urrent models of raciation and speciation.