Demographic aspects of interspecific hybridization : A study of the Jaera albifrons species complex (Crustacea, Isopoda, Asellota).

A demographic study was carried out on two closely related species of the isopods Jaera (albifrons) ischiosetosa and J. (a.) albifrons and their F 1 hybrids. The results from the 16 possible combinations of crosses have permitted an analysis of the nature of the mechanisms assuring the isolation of the species studied. Although intraspecific crosses yield an immediate success, interspecific crosses in the absence of choice of mates progress only slowly during the course of weeks. The results of both crosses between hybrids and back-crosses turn out to be intermediate between those of intra- and interspecific crosses. The hybrids of the first generation are perfectly viable and their survival curves are identical to those of the parents. The fertility of parents in intra- and interspecific crosses is comparable, with the exception of the fragility of female descendants (heterogametic sex) in one direction of crossing. The fertility of the F 1 hybrids, however, crossed either among themselves or with their parents, is quite noticeably decreased: the time needed to double the size of the population is 2.5 times longer for the hybrids than for the parents. This hybrid breakdown completes the pre-fertilization isolating mechanisms: partial ecological isolation, and especially ethological isolation, is practically total when a choice of mates exists. The two species studied, for which demographic parameters are quite close, were raised together for ten generations and yielded only exceptional hybrids with a frequency which does not exceed that found under natural conditions.