Experimental hybridization of alpine and lowland forms of Boeckella dilatata, a calanoid copepod

Abstract

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 transparent while in alpine bodies of water, it is larger and bright red. The life history traits of alpine and lowland populations of this copepod also differ. In this paper we present the results of experimental hybridization between individuals from an alpine and a lowland population. Crosses between a transparent, lowland morph of B. dilatata and a red, 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 backcrossed to lowland males, however, were significantly smaller than those of females in other F1 crosses; in addition, a high variance in clutch size was observed in all F1 crosses, which is also interpreted as reduced fitness. F1 hybrid breakdown was also apparent from the observation that clutch sizes of females in backcrosses with a parental constitution 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 alpine and transparent lowland morphs of this copepod are best regarded as conspecifics in the early stages of speciation and we discuss the results as they relate to current concepts of speciation. F1 and F2 offspring all showed red pigmentation of similar intensity to the original alpine 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.