Developmental time and thorax length differences between the cactophilic species Drosophila buzzatii and D. koepferae reared in different natural hosts JUAN J. FANARA*, JULIAN MENSCH, GUILLERMO FOLGUERA and ESTEBAN HASSON Departamento de Ecologia, Genetica y Evolucion, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria Pab. II. (1428) Buenos Aires, Argentina (*author for correspondence, tel./fax: +54-11-4576-3384; e-mail: jjfanara@bg.fcen.uba.ar) Received 26 May 2003; accepted 2 March 2004 Co-ordinating editor: L.D. Hurst Abstract. Drosophila buzzatii and Drosophila koepferae are two cactophilic sibling species whose ranges partially overlap in Northwestern and Western Argentina. Both species can utilize the decaying tissues of both Opuntia and columnar cacti as breeding sites. Though D. buzzatii and D. koepferae are not differentially attracted to Opuntia and columnar hosts, the composition of the communities of flies emerging from natural substrates of both cacti differed significantly in a natural population. The objective of this paper is to analyze whether intra and/or interspecific competition affects development time and thorax length in D. buzzatii and D. koepferae when both species are reared in single and mixed species culture and fed with semi-natural media prepared with fermenting materials of Opuntia sulphurea (tuna) and Trichocereus terschekii (cardo´n). Our results showed that both traits differ significantly between flies raised in different hosts and that differences between D. koepferae and D. buzzatii species for both thorax length and development time depend on the type of culture (mixed vs. single species). In addition, the host by type of culture interaction was significant. We also observed thorax length differences between Drosophila species and type of culture. Our present data suggest that the effect of intra and interspecific competition varied between the two traits investigated and between species. However, competition alone cannot explain the differential pattern of resource utilization shown by D. buzzatii and D. koepferae in the natural population studied. Key words: body size, cactophilic Drosophila, competition, development time, host plants Introduction The history of the evolution of species interactions is closely related to the history of biodiversity (Thompson, 1999). Diversity can be reduced by com- petitive exclusion in species with similar ecology that live in the same spatial region (Pimm et al., 1985; Krebs and Barker, 1991; Pascual et al., 1998; Morris, 1999). However, the coexistence of competitive species is possible when Evolutionary Ecology 18: 203–214, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.