RESUMEN
Maternally inherited endosymbionts that manipulate the reproduction of their insect host are very common. Aside from the reproductive manipulation they produce, the fitness of these symbionts depends in part on the direct impact they have on the female host. Although this parameter has commonly been investigated for single infections, it has much more rarely been established in dual infections. We here establish the direct effect of infection with two different symbionts exhibiting different reproductive manipulation phenotypes, both alone and in combination, in the fruit fly Drosophila melanogaster. This species carries a cytoplasmic incompatibility inducing Wolbachia and a male-killing Spiroplasma, occurring as single or double (co-) infections in natural populations. We assessed direct fitness effects of these bacteria on their host, by comparing larval competitiveness and adult fecundity of uninfected, Wolbachia, Spiroplasma and Wolbachia-Spiroplasma co-infected females. We found no effect of infection status on the fitness of females for both estimates, that is, no evidence of any benefits or costs to either single or co-infection. This leads to the conclusion that both bacteria probably have other sources of benefits to persist in D. melanogaster populations, either by means of their reproductive manipulations (fitness compensation from male death in Spiroplasma infection and cytoplasmic incompatibility in Wolbachia infection) or by positive fitness interactions on other fitness components.
Asunto(s)
Drosophila/microbiología , Spiroplasma/patogenicidad , Simbiosis/fisiología , Wolbachia/patogenicidad , Animales , Supervivencia Celular , Herencia Extracromosómica , Femenino , Fertilidad , Larva/crecimiento & desarrollo , MasculinoRESUMEN
Elucidation of the mechanism of action of selfish genetic elements is difficult outside species with well-defined genetics. Male-killing, the phenomenon whereby inherited bacteria kill male hosts during embryogenesis, is thus uncharacterized in mechanistic terms despite being common and important in insects. We characterized the prevalence, identity and source of the male-killing infection recently discovered in Drosophila melanogaster in Brazil. Male-killing was found to be present in 2.3% of flies from Recife, Brazil, and was uniquely associated with the presence of Spiroplasma infection. The identity of sequences across part of the 16S and across the 16S-23S ITS region indicated that the male-killing infection of D. melanogaster was very closely related to S. poulsonii, the source of the male-killing infection in willistoni group flies also found in South America. The sequences of two further protein-coding genes indicated the D. melanogaster infection to be most closely related to that found in D. nebulosa, from the willistoni group. Our data suggest that the establishment of D. melanogaster in South America was associated with the movement of male-killing bacteria between species.