INTERSPECIFIC HYBRIDIZATION IN DAPHNIA - DISTINCTION AND ORIGIN OF HYBRID MATRILINES

Three coexisting Daphnia species belonging to the D. longispina group (D. galeata, D. hyalina, and D. cucullata) form species-hybrid complex es by producing interspecific hybrids in several lakes in Germany and The Netherlands. To evaluate the genetic consequences of interspecific hybridization, I studied the patterns of mitochondrial DNA (mtDNA) se quence variation. The directionality of interspecific hybridization an d divergence of hybrids from parental species was tested, using the DN A sequences of a segment of mtDNA. Via the polymerase chain reaction, it was possible to investigate single animals and even single resting eggs. A species-specific marker was established, using restriction pat terns of amplified cytochrome b segments. mtDNA genotypes of hybrids r evealed unidirectional mitochondrial gene flow for two hybrids, which were investigated by using multiple clones. No evidence for introgress ion of mtDNA was found. On the basis of a phylogenetic analysis, the s pecies exhibit considerable distinctness, whereas differences between clones within species and between hybrids and maternal species tend to be very low. These results indicate a recent origin of hybrids and su ggest that the radiation of the D. longispina group occurred >5 Mya.