The Evolutionary History of Drosophila buzzatii. XIII. Random Differentiation as a Partial Explanation of Chromosomal Variation in a Structured Natural Population

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Abstract
Drosophila buzzatii is associated with several cactus species of the genus Opuntia (prickly pear). The rotting cladodes of Opuntia constitute discrete and ephemeral breeding sites colonized each generation by a finite number of mature D. buzzatii females. The genetic consequences of this population structure on the chromosomal variation in a natural population have been investigated by means of F statistics. Several conclusions are drawn from the present study: (1) the average number of parents breeding on a single Opuntia cladode can be estimated as 10; (2) there is a significant within-cladode excess of inversion heterokaryotypes for the second chromosome (since this excess is still significant when the whole population is considered, it cannot be explained solely as a result of the founder effect associated with the colonization of each cladode); and (3) FIS and FST values for gene arrangements are heterogeneous on the second chromosome, which suggests that, for this chromosome, differentiation of inversion frequencies among breeding sites has not occurred at random. These results strongly point to the existence of differential fitnesses (attributable to the viability component of the egg to the third-instar larva) among karyotypes in nature, which agrees with previous analyses. Several assumptions underlying this analysis and the possible type of selection operating in the population are discussed.