Differences in Tolerance to Host Cactus Alkaloids in Drosophila koepferae and D. buzzatii

Abstract

The evolution of cactophily in the genus Drosophila was a major ecological transition involving over a hundred species in the Americas that acquired the capacity to cope with a variety of toxic metabolites evolved as feeding deterrents in Cactaceae. D. buzzatii and D. koepferae are sibling cactophilic species in the D. repleta group. The former is mainly associated with the relatively toxic-free habitat offered by prickly pears (Opuntia sulphurea) and the latter has evolved the ability to use columnar cacti of the genera Trichocereus and Cereus that contain an array of alkaloid secondary compounds. We assessed the effects of cactus alkaloids on fitness-related traits and evaluated the ability of D. buzzatii and D. koepferae to exploit an artificial novel toxic host. Larvae of both species were raised in laboratory culture media to which we added increasing doses of an alkaloid fraction extracted from the columnar cactus T. terschekii. In addition, we evaluated performance on an artificial novel host by rearing larvae in a seminatural medium that combined the nutritional quality of O. sulphurea plus amounts of alkaloids found in fresh T. terschekii. Performance scores in each rearing treatment were calculated using an index that took into account viability, developmental time, and adult body size. Only D. buzzatii suffered the effects of increasing doses of alkaloids and the artificial host impaired viability in D. koepferae, but did not affect performance in D. buzzatii. These results provide the first direct evidence that alkaloids are key determinants of host plant use in these species. However, the results regarding the artificial novel host suggest that the effects of alkaloids on performance are not straightforward as D. koepferae was heavily affected. We discuss these results in the light of patterns of host plan evolution in the Drosophila repleta group.

Pupal emergence pattern in cactophilic Drosophila and the effect of host plants

“…D. buzzatii showed and interesting variation in the time of emergence from pupae between cactus hosts while D. koepferae tended to be more resilient (a pattern previously observed on other traits; Soto et al ., ; Soto et al ., ; Soto et al ., ). Previous observations could be considered in the context of the present results.…”

“…Studies of natural populations in the field and under laboratory conditions have shown that D. buzzatii and D. koepferae differ in their preferences regarding oviposition sites (Soto et al ., ) and higher viabilities, faster developing times and increased mating success for both species were observed when they were reared in their respective primary hosts (Soto et al ., ; Hurtado et al ., ). Here we enlarged the list of traits in which host‐dependent phenotypic plasticity is displayed by D. buzzatii whereas D. koepferae showed a more canalized behavior as previously observed for viability, developmental time (Soto et al ., ,b, , ), starvation resistance, oviposition preference (Soto et al ., ), and genital morphology (Soto et al ., ). As a performance‐related trait, time of pupal emergence was positively correlated with developmental time and negatively correlated with adult wing size.…”

“…WS (as a proxy for body size) was scored following Soto et al . () protocol. Briefly, right wings of adult flies of both sexes were removed and mounted on slides.…”

“…Nonetheless, in the vast areas where both species occur in sympatry, a certain degree of overlap in host exploitation occurs, with the 2 species emerging from both resources despite maintaining a preference for their respective primary host (Hasson et al ., ; Soto et al ., ). Previous studies showed that traits associated to fitness were maximized when flies were reared in their primary hosts (Soto et al ., ,b, , ; Hurtado et al ., ). Developmental instability levels increased when flies of both species were reared in their respective secondary host (Soto et al ., ).…”

Biodiversity of cactophilic microorganisms in western Argentina: community structure and species composition in the necroses of two sympatric cactus hosts

“…With this respect, hostvector and host-microbe chemical interactions in the arid regions of South America may be analogous to those taking place in the Sonoran desert. So far, alkaloids isolated from T. terscheckii were shown to differentially affect fitness related traits in D. buzzatii and D. koepferae (Corio et al, 2013;Padr o et al, submitted;Soto et al, 2014), and samples of T. terscheckii and O. sulphurea have been shown to differ in their relative concentrations of medium-chain fatty acids Padr o and Soto, 2013). These two types of compounds are known to shape patterns of host use by cactophilic Drosophila and yeasts in the Sonoran desert (Fogleman and Danielson, 2001;Fogleman and Heed, 1989), however, their impact in the structure of other similar systems has been seldom considered.…”