Factors affecting the co-existence of the serpentine endemic Mimulus nudatus Curran and its presumed progenitor, Mimulus guttatus Fischer ex DC

Population genetics theory suggests that two species which are reproductive ly isolated only by postzygotic barriers cannot co-exist in sympatry since the rarer population will become extinct Mimulus nudatus and Mimulus guttat us are two closely related species that are isolated by a postygotic barrie r operating at the seed provisioning stage. On several sites on the serpent ine soils of Lake and Napa counties, California, M. nudatus and M. guttatus live in sympatry and flowering times of the species overlap, so the specie s are sometimes flowering next to each other. We investigated whether there was any reduction in fertility of M. nudatus and ill. guttatus caused by i nterspecific crosses when growing in sympatry. The pollinators of M. nudatu s and M. guttatus were identified. Small swear bees, Dialictus sp., prefere ntially visited the smaller flowered M. nudatus species and honey bees, Api s mellifera, preferred the larger flowered M. guttatus. In spire of most po llinator visits being intra-specific, individuals of both bee species made transitions between the Mimulus species. This will result in greater pollen transfer from M. guttatus to M. nudatus than vice versa because firstly, M . guttatus produces more pollen and secondly, the sweat bees were too small to touch the stigma of a M. guttatus flower. This asymmetry in gene flow w as detected by greater reduction in viable seed produced by ill nudatus pla nts when surrounded by M. guttatus plants than vice versa. Only when M. nud atus was the maternal parent could any hybrids be detected in field produce d seed. To enable M. nudatus and M. guttatus to co-exist in sympatry, the t wo species may thus need to be sufficiently ecologically different. Such ec ological differentiation was attributed to the greater drought tolerance of M. nudatus. In addition, greenhouse experiments suggested that M. nudatus may have evolved greater tolerance to calcium deficient soils. (C) 2000 The Linnean Society of London.