Population structure of Rhagoletis mendax, the blueberry maggot

Abstract

Rhagoletis mendax, the blueberry maggot, is a member of the R. ponionella species complex, wellknown as a possible example of sympatric speciation via host race formation. Using electrophoretic data for 16 loci, I have determined that the population structure of R. mendax differs sharply from that of the well-known apple maggot fly, R. pomonella, as follows: (i) geographical differentiation across eastern North America is very small (FsT=O.OlS as opposed to FsT=O.22O in R. pomonella), (ii) no latitudinal dines in allele frequencies exist, and (iii) consistent differentiation with respect to individual host plant and host plant species was not seen (although such differentiation could not be studied as effectively in R. mendax as in R. pomonella). Fum and Had show strong linkage disequilibrium in two northern populations, and weak but significant disequilibrium across the entire geographical range of R. mendax. The genetic uniformity of R. mendax may be a consequence of its restriction to a single host plant species over most of its range.Keywords: allozymes, population genetics, population structure, Rhagoletis, sympatric speciation, Tephritidae. IntroductionFor almost three decades the fruit flies of the Rhagoletis pomonella species group (henceforth pomonella group) have been the focus of debate over sympatric speciation via host race formation, a mode of speciation in which reproductive isolation evolves, in the absence of geographical isolation, during the adaptation of parasites to novel hosts (Mayr, 1963;Bush, 1966Bush, , 1969Bush, , 1974Bush, , 1975Bush, , 1992Futuyma & Mayer, 1980). Bush hypothesized that genetic variants with altered host selection behaviour and host survival ability can segregate out of a parasite population, producing a new population, on a new host, that is at least partially reproductively isolated from the original population on the ancestral host (namely, a host race).The existence of at least one host race formed in historical time, the apple race of R. pomonella (Walsh), is now well supported (Prokopy et a!., 1987;Feder et a!., 1988Feder et a!., , 1990aMcPheron et a!., 1988b;Smith, 1988 .In this paper, I examine the pomonella group sibling species R. mendax Curran (blueberry maggot) at 16 variable allozyme loci to determine whether its population structure differs from that of R. pomonella. I sampled R. mendax sufficiently well to allow the magnitude of geographical differentiation, pattern of geographical differentiation (for example dines), and amount and degree of linkage disequilibrium to be assessed with some confidence. Provisional answers can be given to questions concerning differentiation of 542 1995 The Genetical Society of Great Britain. POPULATION STRUCTURE OF RHAGOLETIS MENDAX 543R. mendax populations on individual host plants, and on different host plant species, but no data on associations between allozyme and life history phenology were obtained in this study. Biology of R. mendaxThe life cycle of R. mendax is typical of most Rhagoletis species (Boiler & Prokop...

Sympatric Host-Race Formation and Speciation in Rhagoletis (Diptera: Tephritidae): A Tale of Two Species for Charles D.

“…For a 17-locus allozyme set resolved for both species, the composite FST value among R. mendax populations across the eastern United States was 0.015 compared to 0.148 for R. pomonella (Berlocher 1995;Berlocher and McPheron 1996). There is no evidence for latitudinal clines in R. mendax (Berlocher 1995).…”

“…There is no evidence for latitudinal clines in R. mendax (Berlocher 1995). In addition, there appears to be no host-associated variation among R. menda;'( populations infesting different Vaccinium host species (Berlocher 1995).…”

“…For years R. mendax was considered a "variety" of the apple maggot (Woods 1915;Patch and Woods 1922;Lathrop and Nickels 1932;Diehl and Prokopy 1986). But allozyme studies have confirmed that R. mendax and R. pomonella are distinct siblings species, as these flies possess "species-specific" alleles at 11 loci Berlocher and Bush 1982;Berlocher 1995). High-and low-bush blueberries were thought to be the native hosts for R. mendax.…”

“…High-and low-bush blueberries were thought to be the native hosts for R. mendax. However, it has recently been found that the fly specializes on another species of endemic blueberry (deerberries, Vaccinium stamineum) through much of the Southeast (Payne and Berlocher 1995a;Berlocher 1995). This discovery has broadened the known geographic range of R. mendax to the extent that the fly is almost totally sympatric with R. pomonella across eastern North America.…”

“…All R. pomonella group species are indigenous to North America, with taxa being either partially or broadly sympatric in their geographic distributions (Bush 1966). Although several pairs of R. pomonella group species can be crossed in the laboratory (Reissig and Smith 1978;Smith 1988aSmith , 1988bSmith et al 1993), these species are genetically diverged in nature and do not appear to hybridize often (Berlocher 1995;Berlocher and Bush 1982;Berlocher et al 1993;.…”