Electrophoretic Mobility and Larval Substrate within Species

Electromorphic variation among populations of Drosophila mojavensis, D. arizonensis and D. longicornis was examined for seven genetic loci. The average electrophoretic mobility for a population was used as the metric. D. mojauensis and D. arizonensis use larval substrates in different parts of their geographic ranges, while D. Zongicornis is more narrowly restricted to different species of the cactus Opuntia in different localities. There is marked electromorphic variation among populations of either D. mojuvensis or D. arizonensis, and the bulk of this variation is accounted for by differences in lava1 substrate. There is somewhat less variation among populations of D. longicornis, and only a moderate portion of this is accounted for by larval substrate differences. There appears to be an association between the taxonomic diversity of the larval substrates and the electromorphic diversity of the Drosophila populations utilizing those substrates. Evidence is reviewed that suggests physiological mechanisms for these possibly adaptive associations. N an earlier paper (RICHARDSON and SMOUSE 1976), we used the average I electrophoretic mobilities of seven multiple-electromorph loci to measure the molecular variation among species of the Drosophila mulleri complex. One of the more tantalizing observations of that paper was a major electrophoretic difference in esterase-C between the “giant cactus” breeders (D. mojauensis and D. arizonensis) and the Opuntia (“prickly pear” cactus) breeders (D. longicornis, D. pachuca, D. propachuca, D. desertorum, D. tira, D. ritae, D. hexastigma, D. aldrichi and D. mulleri). This finding, when coupled with a consideration of possible modes of evolution of electrophoretic differences, suggests that the larval substrates utilized by the flies might bear some causal relationship to the array of electromorphs recovered for certain loci. The purpose of the present paper is to pursue this matter at the intraspecific (interpopulation) level. D. mojavensis and D. arizonensis not only utilize a taxonomically disparate set of larval substrates from those used by other species of the D. mulleri complex, but each exhibits important interpopulation larval substrate variation as well; Supported by ERDA Contract AT-(40-1)4023. NIH Career Development Award GM 47350-05. Department of Human Genetics, University of Michigan, Ann Arbor, Michigan 48104. a Supported by ERDA Contract E(l1-1)-2828. Genetics 85: 141-154 January, 1977 142 R . H. R I C H A R D S O N . P. E. S M O U S E Ah-D M. E. R I C H A R D S O N different genera of cactus are used in different parts of the geographic rangt. In view of the esterase-C difference mentioned above. we wish to determine the relationship between the electrophoretic mobilities of various loci and the larval substrates of different populations within each of these species. W e shall also examine D. longicornis, whose larval diet is restricted to several species of the genus Opuntia, and which constitutes a control. Formally, we wish to answer the following questions. (1 ) Do D. mojnriensis and D. nrizonensis. which utilize different genera of cacti in different regions, show more electromorphic variation among pcpulations than D. longicornis, which utilizes different Opuntia species in different parts of its range? (2) To what extent is electromorphic variation among populations correlated with larval substrate variation among these same populations? The answers to these questions lead to some interesting suggestions concerning the source of much electrophoretic variation among populations. M A T E R I A L S .4ND M E T H O D S