Abstract:
Two chromosomal forms of white-footed mice (Peromyscus leucopus) exist in Kansas, Oklahoma, and northern Texas. These differ by three pericentric inversions on chromosomes 5, 11, and 20. Individuals representative of the northeastern cytotype and the southwestern cytotype were collected from Kansas populations not in contact with each other, and hybrids between the two cytotypes were collected from within the zone of contact in Oklahoma. Breeding pairs were established, and hybrids, whether collected in the zone of contact or produced in the lab, showed no reduction in fertility. Chromosomes of hybrid individuals collected from within the zone of contact in Oklahoma were compared to chromosomes of hybrids formed between mice collected outside the zone of contact. Silver-stained spermatocyte spreads prepared from hybrids produced within the zone of contact and from hybrids produced from populations not formerly in contact were observed during meiosis. These spreads showed heterosynaptic pairing of the inverted regions of the hybrid's chromosomes. No inversion loops were observed. Heterosynaptic pairing, which is interpreted as a mechanism
to avoid hybrid infertility, was not found to be the result of an adaptive mechanism that is unique to individuals within the zone of contact, but one which is inherent within the species itself.
