a. and for the two interspecific competition (mixed species culture) evaluated for Drosophila melanogaster. b. D. simulans. c. and Zaprionus indianus. d. reared in Mangifera indica (mango), Diospyros kaki (caqui) and Pyrus communis (pear). Control indicates the RPI value obtained for each species reared under single species culture. Error bars corresponds to standard deviations.

To evaluate the effect of interspecific competition in each species, we compared the RPI value obtained in the control (single species culture) and both interspecific competition possibilities (mixed species culture). In all cases each vial (replicate) was considered as the experimental unit. The analysis revealed that Z. indianus exhibited a significant lower RPI relative to both Drosophila species when single species culture was evaluated (F= 44.46; df= 2,36; p <0.001, Fig. 1a). Our results also determined that the resources evaluated as well as the species by resources interaction did not present significant differences. Therefore, Z. indianus had the lower RPI independently of the resources evaluated (Fig. 1a). This result is a consequence of the decrease of viability and a slower developmental time (F= 23.09 and F= 22.37, respectively; df= 2,36; p <0.001 in both cases) exhibited by Z. indianus respect to D. simulans and D. melanogaster ( Table I) whereas, wing length showed similar size in all species studied (F= 0.34; df= 2,36; p =0.71). When we evaluated mixed species culture (Fig 1b-d) only D. simulans was negatively affected by interspecific competition (F= 13.76; df= 2,36; p <0.001) since the presence of any of the other two species reduces its RPI (Fig. 1c). Furthermore, under mixed species culture, the treatment factor showed significant differences for all three traits used to calculate RPI, (F=_{23.18, F= 23.58 and F= 13.28, for viability, developmental time and wing length, respectively; df= 2,36; p <0.001 in all cases), being the viability the trait most affected (Table II).}

Table I.

Standard deviation is indicated between brackets. Means sharing different letters in the same column indicate significant differences between species (p <0.05).

Table II.

Standard deviation is indicated between brackets. Means sharing different letters in the same column indicate significant differences between treatments (p <0.05).

All in all, our results suggest that interspecific competition could be one of the causes about the absence of D. simulans, among the emerged species when this species shares the resource with D. malanogaster and Z. indianus (Fanara et al., 2022). On the other hand, the successful colonization of Z. indianus cannot be attributed to interspecific competition. Certainly, the fitness of D. melanogaster, measured through RPI, was not affected when this species reared with Z. indianus. Thus, future studies should address the reason that allowed Z. indianus to successfully expand its distribution in Argentina.