GENETIC ANALYSIS OF THE MORPHOLOGICAL DIFFERENCES BETWEEN TWO INTERFERTILE SPECIES OF HAWAIIAN DROSOPHILA

Most biological species are reproductively isolated from their closest relatives both in nature and in the laboratory. Thus direct analysis of the genetic differences between them is not possible by Mendelian methods. Inferences as to the extent of genetic differences between species can be based on comparative biochemical studies (e.g. Ayala, 1974) or on polytene chromosome sequencing (Clayton et al., 1972). In key instances of certain very close species, however, both these techniques fail to be useful. Morphologically and behaviorally distinct species are known which cannot be distinguished either by allozymes or chromosomes (Carson et al., 1975). Indeed, the biochemical similarity between such obviously distant species as man and chimpanzee have led to the postulating of regulatory rather than structural genes as the basis of the important biological differences between species (King and Wilson, 1975). I report here the results of hybridization experiments between two morphologically and behaviorally distinct species of Hawaiian Drosophila. By using methods to overcome ethological isolation, it is possible to obtain reciprocal hybrids which are fertile in both sexes (Ahearn and Val, 1975). F2's and backcrosses reveal considerable segregating polygenic differences in morphology. These species show close cytological and electrophoretic similarity. Accordingly, I believe that the character segregations observed represent polygenes regulating morphology rather than differences in chromosome sequences or in structural proteins. The species considered in the present work are Drosophila heteroneura (Perkins, 1910) and Drosophila silvestris (Perkins, 1910). They are endemic to the Island of Hawaii where they are sympatric in many localities. They belong to a cluster of homosequential species of Hawaiian picture-winged Drosophila in the planitibia subgroup. Different aspects of this cluster have been the subject of several recent studies. The species constitute one of the end branches in Carson's chromosome phylogeny (Clayton et al., 1972). At the present time, four species are considered to belong to it: differens Hardy and Kaneshiro (1975), from Molokai; planitibia Hardy (1966), from Maui; heteroneura and silvestris from the Island of Hawaii. D. differens, just recently described as a new species, was referred to as planitibialike from Molokai by Ahearn et al. (1974), Craddock (1974a,b) and Johnson et al. (1975). Speciation in this cluster of species has been discussed in Carson (1970), Carson et al. (1970), Carson (1974), Craddock (1974a,b) and is the subject of an extensive work by Kaneshiro (in preparation). D. heteroneura and D. silvestris constitute a case of sympatric species from which fertile hybrids in both reciprocal directions can be obtained in the laboratory with relative facility. Premating barriers are responsible for reproductive isolation between them, as has been shown in a study of their ethological isolation by Ahearn et al. (1974), and discussed by Craddock (1974a,b). The full fertility of male and female laboratory hybrids makes these species suitable for studying the inheritance of species differences.