Linking behavioral ecology with population genetics: insights fromDrosophila nigrospiraculaEDWARD PFEILER1,2, NANCY M. NGO2and THERESE A. MARKOW31Centro de Investigacio´n en Alimentacio´n y Desarrollo, A.C., Unidad Guaymas, Guaymas, Sonora, Mexico2School of Life Sciences, Arizona State University, Tempe, Arizona, USA3Department of Ecology and Evolutionary Biology and the Center for Insect Science, University of Arizona, Tucson,Arizona, USAPfeiler, E., Ngo, N. M. and Markow, T. A. 2005. Linking behavioral ecology with population genetics: insights fromDrosophila nigrospiracula. */ Hereditas 142:1/6. Lund, Sweden. ISSN 0018-0661. Received November 30, 2004. AcceptedDecember 13, 2004Although Drosophila species provide important model systems for evolutionary biology, the ecologies and natural historiesof most species are insufficiently characterized to permit predictions with respect to issues such as population geneticstructure. A notable exception is the group of cactophilic Drosophila endemic to the Sonoran Desert of North America. Oneof these species, D. nigrospiracula , exhibits no population subdivision anywhere in its range. Here we present evidencesuggesting that the timing of mating in relation to dispersal contributes to the panmixia observed in this species.Therese A. Markow, Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA. E-mail:[email protected] of the genus Drosophila exhibit a wide range ofdifferences in the levels of population genetic differ-entiation observed using allozyme electrophoresis andmolecular genetic studies (POWELL1997; SHOEMAKERand JAENIKE1997; MARKOWet al. 2002; HURTADOet al. 2004). An absence of relevant behavioral andecological studies of the majority of these speciesmakes it difficult to interpret the factors responsiblefor the presence or absence of population structure.Four species of cactophilic Drosophila endemic to theSonoran Desert of North America, however, havebeen well characterized with respect to their ecology(HEED1978, 1982; BREITMEYERand MARKOW1998)and behavior (MARKOW1982, 1988; MARKOWandCASTREZANA2000). All four species have specializedon different columnar cactus host species, feeding andbreeding in necrotic plant tissue or in soil soakedwith the necrotic exudates (HEED1978, 1982). Thefour Drosophila species show contrasting patternsof population subdivision, permitting hypothesesto be tested concerning the relationship betweenecology and behavior and population genetic differ-entiation.One of the cactophilic species, D. nigrospiraculaPatterson & Wheeler, exhibits no population structureacross its range, compared to the other three desertendemic species, all of which exhibit some degree oflocal genetic differentiation. These patterns havebeen found whether the loci used were allozymes(SLUSS1975; PFEILERand MARKOW2001; MARKOWet al. 2002) or molecular genetic markers (HURTADOet al. 2004), and for D. nigrospiracula , gene frequen-cies have been found to be stable for over 30 years(SLUSS1975; PFEILERand MARKOW2001).Potential explanations for the observed lack ofpopulation structure and the temporal stabilityof gene frequencies lie in the behavioral ecology ofD. nigrospiracula. Field studies demonstrate thatcompared to the other desert Drosophila, D. nigrospir-acula is a strong disperser (JOHNSTONand HEED1976; MARKOWand CASTREZANA2000). In thelaboratory, flies of D. nigrospiracula have been foundto become sexually mature later than those ofD. melanogaster: females mate at four and males atsix days of age (MARKOW1982). Reproductive beha-vior has been found to differ in other Drosophilaspecies, however, when field and laboratory studieshave been compared (MARKOW1988, 2000; SNOOKand MARKOW2001). Therefore, while it may betempting to speculate that long distance dispersalcoupled with late reproductive maturity could explainthe apparent extensive gene flow in D. nigrospiracula ,caution must be exercised in the absence of informa-tion about when flies in nature mate relative to whenthey disperse and oviposit. For example, if D. nigros-piracula mate and oviposit prior to dispersing, adultflies emerging from a given breeding patch wouldexhibit some degree of inbreeding, as observed in theOpuntia-breeding D. buzzatii (BARKERand MULLEY1976). On the other hand, if flies disperse before theymate, not only should local inbreeding be insignif-icant, but it would promote a lack of populationHereditas 142: 1/6 (2005)structure. These different scenarios can be distin-guished by field studies.Here we ask two questions about D. nigrospiracula:1) when do flies mate relative to when they disperse?2) is there evidence of inbreeding at a given micro-habitat patch? We address these questions throughcapture-mark-release-recapture studies using virginflies of different ages and with allozyme electrophor-esis of resident adults and newly emerged flies at thesame patch.MATERIAL AND METHODSCapture-mark-release-recapture experimentsExperiment 1. */ This initial experiment was designedto provide an estimate of the proportion of matedfemales that would be expected to be utilizing a micro-habitat patch in the wild and to obtain an estimate ofoverall recapture rate of marked males and females.Approximately 1450 wild adults of D. nigrospiraculaof unknown age were collected in January 1997 from asingle large necrotic arm of a saguaro cactus (Carne-giea gigantea) located at the Superstition study site(BREITMEYERand MARKOW1998) east of Tempe,Arizona (33822?N; 111822?W). Flies were taken tothe laboratory, separated by sex under carbon di-oxide anesthesia, counted and then marked with amicronized fluorescent dust (Radiant Corp., Rich-mond, California) as described in MARKOWandCASTREZANA(2000). The following day the markedflies (652 females and 787 males) were released at thesame rot from which they were collected. After 24 hthe rot was revisited and an exhaustive collection ofadult flies was made with a sweep net and by mouthaspiration. We continued collecting until no adult fliescould be detected on the rot. Females and males wereseparated in the field, returned to the laboratory, andscored for the presence of fluorescent dust. Twenty ofthe recaptured marked females were dissected to checkfor the presence of sperm in their ventral receptacles.Experiment 2. */ To obtain information on age andmating status of female D. nigrospiracula, approxi-mately 2000 flies were reared in the laboratory onpotato/saguaro