BOTTLENECK EFFECTS IN LOCAL-POPULATIONS OF FOSSORIAL CTENOMYS (RODENTIA, CTENOMYIDAE) AFFECTED BY VULCANISM

Bottleneck effects in three local populations of the fossorial rodent Ctenomys maulinus brunneus as caused by the recent eruption of the Lon quimay volcano in the Andes of Southcentral Chile are presented. Compa rative census estimates in Rio Colorado indicated a 98.3 per cent decr ease in the breeding population size after the catastrophe. All parame ters of genetic diversity were drastically affected and surpassed neut ral expectations in each population. The proportion of polymorphic loc i decreased by 57 per cent, 100 per cent and 83.2 per cent in the bott leneck populations of Rio Colorado, Las Raices and Alto Bio Bio, respe ctively. The source populations were estimated to have 2.2, 1.5 and 1. 4 alleles per locus whereas the three derived populations had estimate d values of 1.4, 1.0 and 1.1, respectively. Average heterozygosity dro pped by 71 per cent, 100 per cent, and 57 per cent in the same populat ions, respectively. The spatial genetic structuring observed before th e eruption indicated a high degree of population subdivision (Wahlund effect), consistent with an isolation-by-distance model. After the eru ption, excessive microspatial genetic differentiation and larger-scale homogeneity indicated drastic disruption of the breeding or social un its. Low levels of genetic variation in Andean Ctenomys, claimed to be an adaptive response to the stable subterranean niche, can be attaine d by the recurrent catastrophe-induced effects of genetic drift.