EXPERIMENTAL HYBRIDIZATION OF ALPINE AND LOWLAND FORMS OF BOECKELLA-DILATATA, A CALANOID COPEPOD

The freshwater calanoid copepod Boeckella dilatata Sars inhabits lakes and ponds of glacial origin in central and southern South Island, New Zealand. In lowland lakes and ponds, this copepod is small and transp arent while in alpine bodies of water, it is larger and bright red. Th e life history traits of alpine and lowland populations of this copepo d also differ. In this paper we present the results of experimental hy bridization between individuals from an alpine and a lowland populatio n. Crosses between a transparent, lowland morph of B. dilatata and a r ed, alpine morph produced viable F1. Intermorph matings showed no sign of reduced clutch size in either alpine or lowland females. Back- and inter-crossing of the F1 showed that this generation was fertile and that subsequent F2 were viable. Clutch sizes of F1 hybrid females back crossed to lowland males, however, were significantly smaller than tho se of females in other F1 crosses; in addition, a high variance in clu tch size was observed in all F1 crosses, which is also interpreted as reduced fitness. F1 hybrid breakdown was also apparent from the observ ation that clutch sizes of females in backcrosses with a parental cons titution of 25 per cent alpine were significantly smaller than clutch sizes of females in pure crosses. These observations are attributed to heritable differences in life history characteristics between alpine and lowland populations of B. dilatata. We conclude that the red alpin e and transparent lowland morphs of this copepod are best regarded as conspecifics in the early stages of speciation and we discuss the resu lts as they relate to current concepts of speciation. F1 and 2 offspri ng all showed red pigmentation of similar intensity to the original al pine parental generation and we conclude that either pigmentation in B . dilatata is a polygenic character with dominance or is induced by a factor present in the alpine water.