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Interspecific Competition in the Laboratory between Drosophila subobscura and D. azteca.

MARTA PASCUAL [1]

ABSTRACT.--Drosophila subobscura and D. azteca are closely related species that have co-existed on the west coast of North America since that area was colonized by D. subobscura in the late 1970s. We have studied competition between the two species by the serial transfer technique at different initial proportions (20%, 50%, 80% and 100%) and at two densities (20 and 100 individuals). The cultures were maintained at 18 C. In the mixed cultures D. subobscura outcompeted D. azteca at both densities and all initial proportions. Survivorship was similar for both species, although the productivity of D. azteca was reduced, especially in the mixed cultures. The productivity of D. subobscura in the mixed cultures was a function only of the initial number of individuals of this species, and was completely unaffected by the number of D. azteca in the populations. Carrying capacity was attained by both species when the number of founder individuals was twenty; however, the number of descendants was much lower for D. az teca than for D. subobseura. In the pure cultures an important effect of season was detected in the replicates: the productivity of D. subobscura was much higher in spring than in autumn, while the reverse was observed in D. azteca. This effect was not detected in the mixed populations. Thus, lower carrying capacity, lower fecundity, longer mean eclosion rate, delayed oviposition or a combination of all of them could explain the low success of D. azteca in the mixed cultures.

INTRODUCTION

Drosophila subobscura, an endemic Palearctic species, colonized South and North America in the late 1970s. The origin of the colonizers remains unknown, but previous studies suggest that the colonization of both hemispheres derives from the same original population (Latorre et al., 1986; Rozas and Aguade, 1991; Mestres et at., 1992; Balanya et al., 1994). The colonization was detected at its beginning and thereafter the species spread very quickly in both hemispheres (Prevosti et al, 1989). The colonized areas are climatically very similar, but in South America D. subobscura is the only species of the obscura group, whereas several species of this group are endemic to North America. The Nearctic species of the obscura group are classified into two subgroups (Beckenbach et al., 1993): the pseudoobscura and the affinis subgroup. The only representatives of the pseudoobscura subgroup coexisting with the colonizing species are D. pseudoobscura, D. persimilis and D. miranda, all of them restricted to the western side of North America (Buzzati-Traverso and Scossiroli, 1955). Drosophila azteca and the western-northern race of D. athabasca are the only representatives of the affinis subgroup coexisting with the colonizing species (Johnson, 1984; Pascual et al., 1997). Drosophila subobscura shows a north-south dine of abundance along the west coast of North America, being more abundant in the north, with a sharp discontinuity in the north of California and southern Oregon, where the boundaries of the distribution areas of D. azteca and D. athabasca overlap (Pascual et al, 1997).

The expansion of D. subobscura in North America has continued; lately it has been collected farther east (Noor et al 1998). Knowing whether D. subobscura is interacting with other similar species and how this affects the dynamics of the populations might help to predict the progress of range expansion. It has been shown (Pascual et al., 1993) that the seasonal patterns of D. subobscura and those of the endemic obscura group species coexisting with it in North America are similar. Interspecific laboratory competition between D. subobscura and D. pseudoobscura, the most widespread and abundant species of the pseudoobscura subgroup, showed that the colonizing species was a poor competitor since it was eliminated from all competition cultures in a few generations regardless of the temperature and initial frequency used (Pascual et al., 1998).

In nature, the frequency of Drosophila pseudoobscura steadily decreases towards the north and D. azteca is rarely found from the north of California onwards, coinciding with the increase of D. subobscura. Thus, D.azteca could be important in the establishment of D. subobscura in the southernmost part of its distribution where these two species coexist in rather low densities and D. pseudoobscura is the most abundant species. The competition of the three species together could not be analyzed because the females of D. azteca and D. pseudoobscura are not morphologically distinguishable and inferences of the proportion of the two species from the observed proportion of males is not reliable because the sex ratio in D. pseudoobscura is biased towards females.

Thus, in the present work we studied adult competition between Drosophila azteca and D. subobscura at different frequencies and densities that might favor one or the other, or facilitate their coexistence in the laboratory. Finally, we have analyzed different fitness components, such as survivorship, productivity and rate of development to ascertain which ones might be responsible for the success of the species in the laboratory experiments and help us to understand the outcome of the invasion of D. subobscura.

MATERIALS AND METHODS

Strains of flies.--The strains of Drosophila subobscura and D. azteca used in the present work were collected in April 1994 in Gilroy, California (37[degrees]00 'N, 121[degrees]34'W). Males of these species are easily identified by the position and tooth number of their sex combs. Obseura group females are more difficult to distinguish; the enlarged sensory bristles on the leading margin of the wings of all the endemic Nearctic obscura group species do not exceed a third of the distance between the ends of the second and third veins whereas they usually extend to the mid point in D. subobscura (Beckenbach and Prevosti, 1986). These two morphological characters were used to classify all the descendants in the competition experiments.

Each strain was established from the progeny of 25 isofemale lines maintained in the laboratory at 18 C for 10 mo. The strains were subsequently maintained by mass culture.

Competition experiments.--Competition between Drosophila subobscura and D. azteca was studied by the serial transfer technique Type 2 (Ayala et al., 1973), because we wanted to compare the present results with those obtained in a previous competition study between D. subobscura and D. pseudoobscura using the same technique (Pascual et al., 1998).

A specified number of adults (consisting of equal numbers of males and females) were left in a 140 ml bottle with 35 ml of fresh Drosophila medium (41.5 g yeast, 10 g agar, 15 g sugar, 142.5 g corn flour, 2 g Nipagin and 5 ml propionic acid for 1 L water). The age of the flies introduced into the bottles ranged from 2 to 14 d. After 1 wk the survivors were counted and discarded. The adults emerging from the culture were then recorded over the following 6 wk to obtain the complete first-generation. The number of adults emerging constituted the productivity of the culture; and that divided by the number of founder females constituted the productivity per female. Single and mixed populations were started at low (20 individuals) and high (100 individuals) density. The cultures were maintained at 18 C and constant light. The mixed cultures were initiated at three different frequencies, 20%, 50% and 80% of D. subobscura, and the complementary frequencies of D. azteca. All experimental conditions using different de nsities and frequencies were performed simultaneously and a total of eleven replicates were carried out for each experimental condition. For practical reasons, the replicates were grouped in two blocks: from May to June seven replicates were carried out (spring replicates) and four replicates from October to November (autumn replicates). Competition experiments during summer were avoided due to prior observation of the reduced performance of laboratory stocks during these months.

The developmental time from egg to adult for Drosophila subobscura at 18 C is 23 d, although high density increases the length of the period (Orengo and Prevosti, 1994). Thus, we counted the number of individuals that had emerged from day 24, after the cultures were founded, to day 42 and did not proceed further to avoid overlapping generations. During the first week all descendants were counted daily; afterwards they were counted two or three times a week. The rate of eclosion was characterized by determining the number of newly emerging flies per day in order to estimate the eclosion rates of the two species

The data were transformed in order to meet the assumptions for the analysis of variance. The arcsine transformation was applied to the percentage survivorship, and the square root transformation to the number of descendants per female (Sokal and Rohlf, 1995). The probabilities were adjusted by a sequential Bonferroni technique (Rice, 1989). Productivity and survivorship probabilities were analyzed separately.

RESULTS

No significant differences in survivorship between species were observed in the single or in the mixed species cultures (Table 1). In contrast, productivity per female differed significantly between species, densities and frequencies in both single and mixed cultures (Table 2). In single species cultures, productivity of Drosophila subobscura was approximately 40% higher than that of D. azteca, and both were reduced 80% at high density. The productivity per female of D. azteca was very much reduced in mixed cultures (Table, 1). Differences among initial frequencies were also significant; nevertheless, the significant effects of the species-by-frequency interaction were only due to differences in D. subobscura. Productivity per female was density dependent; the species-by-density interaction was significant due to a severe reduction in the number of descendants of D. azteca at high density.

Two different conditions have been represented with the same initial number of individuals (20): the single low-density cultures and the high density mixed cultures when the frequency of the species was 20%. As shown in Figure 1, the mean number of descendants of Drosophila subobscura in these two conditions was so similar that both dots coincide. When those two sets are compared there are no significant differences for productivity per female (t 0.192, P = 0.849) nor for survivorship (t = 0.446, P = 0.661).

When the initial number of individuals in the bottles was twenty, the maximum number of descendants was obtained for both species. The carrying capacity of Drosphila subobscura ([approximate]500 individuals) was higher than that of D. azteca ([approximate]300 individuals) (Fig. 1).

The season effect was analyzed in the single cultures where the replicates were grouped into spring and autumn (Fig. 2). Interestingly, the mean number of descendants of Drosophila subobscura was higher in spring than in autumn, while the reverse was observed in D. azteca. Differences between seasons were analyzed by means of an analysis of variance after square root transformation of the number of descendants. A significant season effect by species interaction was detected at low (F = 12.84, P [less than] 0.005) and high density (F = 5.44, P [less than] 0.05). However, when each species was considered separately differences were only significant at low-density for D. azteca (F = 10.24, P [less than] 0.05) and at high density for D. subobscura (F = 5.18, P [less than] 0.05). No differences between seasons were observed in the mixed cultures.

Drosophila subobscura descendants started emerging earlier and finished later than D. azteca. This behavior was observed in the single cultures at both densities and in all replicates. In crowded cultures developmental time increased and emergence started later and extended further. No differences were observed in the daily emergence of D. subobscura in the mixed species cultures (Fig. 3), the presence of D. azteca did not affect their time of development and differences seemed to be only a function of the number of founder individuals of D. subobscura. At high density, since all the cultures had attained their carrying capacity, the pattern of daily emergence was similar. Under these conditions a secondary peak was observed which could not represent a second generation due to the short interval (11 d) since the first individuals emerged. The daily emergence of D. azteca in mixed cultures could not be analyzed due to the low number of descendants produced by this species, with resulting high differences betw een replicates.

There was a positive correlation between the number of Drosophila subobscura individuals founding the cultures and the mean time of development (r = 0.57, P [less than] 0.001). There was also a significant correlation between the number of founders and the SD of developmental time, which is used as a measure of variability (r = 0.51, P [less than] 0.001). D. subobscura presented seasonal differences in mean time of development (F = 28.35, P [less than] 0.001), and in its SD (F = 8.30, P [less than] 0.01), both of them with higher values in autumn replicates. Autumn replicates of D. azteca also had a higher mean time of development (F = 6.35, P [less than] 0.05), but similar SD than the spring replicates.

DISCUSSION

In the conditions of the experiments, Drosophila azteca was less productive and a poor competitor in relation to D. subobscura. The low number of descendants per female produced by D. azteca in the cultures was probably a result of poorer adaptation to laboratory conditions, lower fecundity or lower carrying capacity. Furthermore, the productivity of D. azteca was enormously decreased in mixed cultures where speed in oviposition and rate of development probably were important. On the contrary, the productivity of D. subobscura in the mixed cultures was totally unaffected by the presence of D. azteca behaving as if it was alone the density of the cultures being only a consequence of the number of D. subobscura founders.

The higher number of descendants emerging in the spring replicates of Drosophila subobscura and the higher number of descendants emerging in the autumn replicates of D. azteca could be the result of seasonal fluctuations in these species; i.e., our results in the autumn replicates of D. subobscura could be explained as a by product of their drastic decrease in summer. In D. subobscura, seasonal fluctuations in laboratory populations had been previously reported (Nogues, 1977; Pascual, 1993). In the present work, for the first time, seasonal differences in productivity have been reported for D. azteca. Fry et al. (1996) stated that productivity shows highly significant genotype by environment interactions. Selection in favor of individuals increasing productivity or mating activity (Pascual et al., 1990) would explain the increase of D. azteca descendants related to the time of maintenance in the laboratory. Nevertheless, no improvement in the productivity of D. azteca was recorded in the autumn replicates of the mixed cultures although all the conditions were repeated simultaneously. More experiments in different years should be carried out in order to be sure that D. azteca has seasonal fluctuations.

Under strong competitive stress, third instar larvae, regardless of their genotype, can arrest their development, resuming it once the competition has relaxed (Mensua and Moya, 1983; Gonzalez-Candelas et al., 1990). The secondary peak of emergence observed for Drosophila subobscura in our high density cultures (Fig. 3) could be due to larval arrest, shown to be a common phenomenon produced in crowded conditions (Botella et al., 1988; Orengo and Prevosti, 1994). In contrast, secondary peaks are absent in D.azteca, which indicates that this species would be unable to avoid competition by larval arrest. In nature, development arrest could be involved in the maintainence of the populations in changing environments since adverse environmental conditions could be avoided. Development arrest caused by scarcity of food resources might have similarities to photoperiodic diapause induced in some species. Since there is no effect of photoperiod in D. subobscura (Lankinen, 1993) larval arrest could be giving an advantag e in the maintainance of this species if there is density dependent competition in nature.

Speed of development is certainly a component of competitive ability in laboratory populations of Drosophila. A species with a relatively short developmental time should eventually replace a more slowly developing species in a mixed population. In our results, D. azteca, with longer time of development, produces a reduced number of descendants when in competition with D. subobscura, with a shorter time of development. Laboratory strains maintained at their carrying capacity evolve increased larval-to-adult viability, larger body size and longer developmental time (Bierbaum et al., 1989). The strains used in the present study evolved an increased mean time of development probably as a result of being maintained by mass culture and thus at high larval densities. The autumn replicates of both species had a longer mean developmental time, however, this increase did not affect the outcome of the competition.

Competition in laboratory conditions between Drosophila subobscura and D. pseudoobscura showed that the colonizing species was at a clear disadvantage in comparison to the endemic nearctic species since its productivity was decreased in mixed cultures (Pascual et al., 1998). The comparison of the results obtained in the present work with those obtained in that previous study indicate that, of the three obscura group species analyzed, D. pseudoobscura is the best competitor. The fecundity of D. pseudoobscura is the highest (Nickerson and Druger, 1973); with respect to D. subobscura this seems to be the only differential trait, since both of them presented similar adaptation to laboratory conditions and similar carrying capacity. Drosophila azteca is the worst competitor, probably due to a combination of factors such as longer developmental time, longer initiation of oviposition in the cultures, lower fecundity, lower carrying capacity and a worse adaptation to laboratory conditions among others. As these spec ies do coexist in nature, mechanisms that prevent competition should be considered such as priority effects on a patchy environment (Shorrocks and Bingley, 1994), intraspecific aggregation over patches (Sevenster and van Alphen, 1996), differences in life history of the species (Sevenster and van Alphen, 1993; Loreau and Ebenhoh, 1994), use of different parts of the process of decay of the same substrates (Nunney, 1990; Krebs and Barker, 1991) or the fact that the communities are not fully saturated (Shorrocks and Sevenster, 1995). The future outcome of the colonization will show the relevance of the results obtained in the laboratory experiments.

Acknowledgments.--This work was supported by grant PB96-0793-C04-03 from the DGES, Spain. We thank F. J. Ayala and R. Huey for their helpful comments and suggestions.

(1.) Corresponding author, e-mail: mpascual@porthos.bio.ub.es

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Author:PASCUAL, MARTA; SAGARRA, ELISABET; SERRA, LUIS
Publication:The American Midland Naturalist
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Date:Jul 1, 2000
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