Z. 2001. Syst. Evo1ut.-forsch. 32 (1994) 44-50 0 1994 Verlag Paul Parep Hamburg und Berlin ISSN 0044-3808

Received on 25. May 1992

Department of Genetics, Faculty of Biology, University of Barcelona, Spain

Preadult competition between Drosophila subobscura and Drosophila pseudoobscura

By D. J. ORENGO and A. PREVOSTI

Abstract

The Palaearctic species DYOSO hika subobscura has recent1 colonized a large area of North

of these s ecies were studied at 13"C, 18°C and 23°C and at densities of 10,50, 100 and 200 eggs per vial. %he two species were differently affected by density and-temperature in the ran es studied. Both intra- and interspecific cultures showed that D . pseudoobscuru was best adaptecfto 23 "C, where it was clearly the dominant s ecies. On the other hand, at 18 "C and es ecially at 13"C, althou h D. subobscura was less viabye than D. pseudoobscuru, its developmentaytime was shorter, whict may give advantage to this species. Results reported here agree with the observed distribution of these species in North America.

Key words: Developmental rate - Drosophika pseudoobscuru - Drosophila subobscuru - Inter-

America where it coexists wit R Drosophila pseudoobscuru. 6 e viability and developmental rate

specific competition - Viability

Introduction

Drosophtka pseudoobscura and Drosophtka subobscura are two species of fruit flies that are included in the D. obscura group. They are morphologically very similar, especially the females. Although their ecology is not well known, these species are probably the most widely used subjects of studies in population genetics, the former by American authours and the latter by Europeans.

Drosophila pseudoobscura occurs in North America but has some isolated populations in Central America and Colombia. Drosophikz subobscura is a Palaearctic species that has recently colonized large areas of both North and South America. In fact, D. subobscura has spread quickly in Chile: three years after its first detection, it could be found from La Serena (29'55's) to Punta Arenas (53"10'S), and it has now become the most abundant Drosophila species within this geographic range (BUDNIK and BRNCIC 1982).

Analyses of D. subobscura populations from both colonized areas suggest a common origin of their founders (BECKENBACH and PREVOSTI 1986; MESTRES et al. 1990; ROZAS and A G U A D ~ 1991). In addition, the environment that was found by D. subobscura in North America was similar to that in South America. Both areas are limited by the Pacific Ocean on one side and a large range of mountains, which in some cases can be passed by flies, on the other side, and they also have similar climates. However, there are several species from the D. obscura group among the autochthonous North American species, while this is not the case in South America. These autochthonous species, among which the most abundant is D. pseudoobscura, could have been expected to prevent D . sub- obscura from expanding and establishing in this area. Nevertheless, 3 years after it was detected, D . subobscura seems to be well established (BECKENBACH and PREVOSTI 1986) although its distribution range in North America is not as wide as in South America; i. e. it is not found South of Ojai (34'28"). Differences in the expansion success of D. sub- obscura in the two hemispheres might be mainly due to interspecific relationships, since

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Preadult competition between Drosophila subobscura and D . pseudoobscura 45

the physical environment and the genetic variability of the founders seem to be very similar.

Competition is certainly an important factor of interspecific relationships. The word “competition” has been used by many authors in different fields of study, with different meanings. So, it is important to emphasize that we use the word “competition” as defined by BIRCH (1957): “Competition occurs when a number of animals (of the same or of differ- ent species) utilize common resources the supply of which is short; or if the resources are not in short supply, competition occurs when the animals seeking that resource neverthe- less harm one or other in the process.”

In this paper we study larval competition between D. subobscura and D. pseudoobscura in the laboratory. Although competition between these species has not been demonstrated in nature, there are some indications that suggest this possibility; e. g. they are closely related species living in the same area, they are attracted by the same baits and, in the laboratory, they grow well in the same medium. In addition, D. pseudoobscura is the most abundant species of the D . obscura group species in its area and if D . subobscura is com- peting with other Drosophila species there, D . pseudoobscura would be the most likely candidate.

Material and methods

We focused in this study on preadult competition. A certain number of eggs were transfer- red to fresh vials and then, the emerging adults were recorded daily This record provided the data to calculate the egg-to-adult viability and the developmental rate.

We studied both intra- and interspecific competition. In order to study the intraspecific competition we set up pure cultures of each species separately at diverse densities and tem- peratures. To study the interspecific competition we set up mixed cultures with 50 % of each species at the same density and temperature conditions as the pure cultures.

Each strain used was started with flies from at least 25 different isofemale lines. We used one strain of D. pseudoobscura and another of D . subobscura captured simultaneously in Gilroy (California) in December 1984. This fact is important since we know from that, that these two strains had a common environment in the past, although for less than 10 years (PREVOSTI et al. 1988). In addition, the fact that the strains had been captured re- cently in nature is of special interest since it assures us that they have not adapted to the conditions of the laboratory (ANDERSON 1973; POWELL 1974; PASCUAL et al. 1990; QUIN- M A and PREVOSTI 1990).

Strains were kept a t 18°C in population bottles with a cornmeal-agar based medium with some Nipagin added. Before starting the experiments cultures had been prepared in which the larval density had been controlled by transferring only 100 eggs per population bottle. These cultures provided a set of flies which had developed in a homogeneous envi- ronment without competitive stress affecting the fitness of the offspring. As flies were emerging we removed them and kept males and females separately for 8-9 days. Then the two sexes were put together and allowed to mate. After 4-5 days the flies were transferred to oviposition boxes. Thus, all eggs were obtained from homogeneously aged parents. Furthermore, this age (12-14 days) corresponds to the major egg-laying time of D. subob- Sara females (NOGUES 1976).

Both intra- and interspecific competition were studied in a total of 12 experimental conditions combining 4 density classes (10, 50, 100 and 200 eggs per vial) and 3 tempera- tures (13, 18 and 23”C), chosen because both species are able to develop within this tem- perature range more or less successfully In each case a total of 200 eggs from each species was used. So, we had a different number of vials depending on density (e. g. in each “single species cultures” at density 10 we had 20 vials and at density 200 we had 1 vial). In the “mixed cultures” we had twice as many vials as in single species cultures (e.g. we had

46 D. J. Orengo and A. Prevosti

1 vial of 200 eggs in single species cultures and 2 vials of the same density in the mixed cultures). The whole experiment was repeated three times in three replicas.

The significance of the different statistical tests, t and F, for developmental time were assessed using approximate randomization tests, which avoids the assumption that vari- ables are normally distributed (for a review of the method see NOREEN 1989).

I

Results and discussion

Intraspecific competition

Density and temperature affected the viability in different ways in the two species as can be seen from Table 1. In general, viability decreased as density increased, but the viability of D. psercdoobscuru was very similar at the three lowest densities. O n the other hand, the differences depending on temperature showed different trends at different densities. The viability of D. subobscura did not change much with temperature. The viability of D. pseudoobscura at 13 and 18°C was more or less the same, but it was considerably higher at 23 "C. The analysis of variance of Table 2A shows a significant effect of density on viability in D. subobscura while temperature does not significantly affect it. In contrast, an ANOVA of the results obtained with D. pseudoobscura (Table 2B) shows a significantly effect of temperature but not of density

Table 1. Relative viabilities of D. subobscura and D. pseudoobscura in intra- and interspecific competition experiments at different densities and temperatures

D. rubobrcura D. pseudoobscura

Alone I Competing I x 2 I Alone I Competing I x'

I Source of variation 1 df I Meansquare I F

10 64.67 50 58.83

l3 100 52.66 200 42.83 10 62.00 50 50.83

200 39.16 10 70.66 50 57.00

23 100 41.00 200 36.50

l8 100 39.33

P

78.00 60.83 50.17 19.50 72.50 61.00 54.16 36.16 66.00 59.00 51.16 43.00

26.08""" 0.50 0.75 76.13""" 15.02""' 12.58""" 26.52""" 1.15 3.02 0.49 12.48""" 5.29"

57.00 60.50 55.80 48.50 55.80 52.83 56.60 45.66 77.66 70.50 74.50 69.83

78.83 65.63'"" 73.00 21.12""' 70.33 27.08"'*" 54.66 4.57" 75.50 51.46""" 70.00 37.30""" 66.66 12.69""" 59.33 21.47*"" 73.33 3.04 68.16 0.76 60.50 26.80""' 53.66 33.19"""

600 eggs of each species were transferred to each experimental condition. Each xz has 1 df. " 0.01 < P < 0.05, "" 0.001 < P < 0.01, *')" P < 0.001.

Density A 3 4985.88 9.98 < 0.005

Temperature A 2 574.08 1.15 B 3 571.51 0.68

B 2 5892.11 7.01 < 0.005

I D x T A 6 209.27 0.42 B 6 102.26 0.12

Error A 24 B 24

Preadult competition between Drosophila subobscura and D. pseudoobscura 47

~~ ~

"C

Table 3. Developmental time (in days) of D. subobscura and D. pseudoobscura in intra- and interspecific competition at different densities and temperatures

D. subobscura D. pseudoobscura

Alone Competing t Alone Competing D

t

Developmental times proved much affected by both, crowding and temperature. In the two species an increase of density or a decrease of temperature caused lengthening of developmental time (Table 3, columns for each species separately). This is significant (p = < 0.05), as the analyses of variance of Table 4 show. The lower the temperature, the greater the effect of density This suggested an interaction between density and tempera- ture, which in the case of D. pseudoobscura was significant (Table 4B).

Emergence began more or less at the same time at all densities, but emergence time extended considerably at higher densities. Figure 1 shows the daily percentage of emergence at 13 "C where this pattern was more clear. The period of emergence was com- pleted by about the 10th day at density 10, while at density 200 it was not finished until 30 days in D. subobscura and 39 days in D. pseudoobscura. We also observed that while at density 10 emergence had a unimodal distribution, at density 100 it had a slight rise near the end, and at density 200 it had various peaks as well as a long tail at the end (about 20 days) in which emergence was very low (1 or 2 flies each day).

The differences between the two species in their density-dependence seem to suggest that density would begin to affect the viability of D. pseudoobscura at higher density values than those that affect D. subobscura. This is probably due to the fact that D. pseudoobsncva is smaller and may need less food to develop completely. O n the other hand, from differ- ences among temperatures we can see that D. pseudoobscura is better adapted to higher

Table 4. Analysis of variance for developmental period of D. subobscura in its pure cultures (A) and analysis of variance for developmental period of D. pseudoobscura in its pure

cultures (B)

Density A 3 12.24 10.50 < 0.05 B 3 51.25 34.24 < 0.005

B 2 1784.45 1192.09 < 0.005 D x T A 6 1.76 1.51

B 6 13.58 9.07 < 0.005 Error A 24

B 24

Temperature A 2 1059.08 908.80 < 0.01

D. J . Orengo and A . Prevosti 48

*/.

40

30

20

10

30

20

10

A _ _ _ _ _ _ DENSITY 200 loo

-.-.-._ 10

B

1 1. i ! i ! ! ! I I

30 40 50 60 + 70

temperatures; this can be also in- ferred from its spatial and tem- poral distribution in nature (PRE- VOSTI et al. 1987), and from la- boratory experience where D. pseudoobscura is normally bred at 25"C, while D. subobscura pre- fers lower temperatures.

The emergence pattern in the crowded cultures suggests that, in addition to individual variability, there must be some phenomenon which causes a periodic variation in emergence. This pattern might be due to the "stopped develop- ment" phenomen described in D. melanogaster by M ~ N S U A and MOYA (1983). These authors found that in high-competition condi- tions some larvae stop developing once the third instar is reached. If conditions change, the stopped larvae can complete their devel- opment in a more or less constant time. The secondary peaks and the tail end of our resulk might cor- respond to those stopped larvae

Fi . 1. Percentage of daily emergin flies at 13°C and at that were able to complete their different larval densities. (A = 8. subobscura, B = development when competitive D. pseudoobscura) conditions relaxed after the non-

stopped larvae began pupation.

Interspecific competition

From Tables 1 and 3 it can be seen that the outcome of interspecific competition experi- ments was different for each set of environmental conditions considered. D. subobscura increased its viability except at the highest density at 13"C, at which its viability was abnor- mally low (only 19.5 % of eggs attained the adult stage). Its time of development increased except at 13 "C and 18 "C at the lowest densities. At 23 "C D. pseudoobscura underwent a reduction in viability and developmental rate. O n the other hand, at 18°C and 13°C its viability was increased but its developmental time was lengthened.

In the mixed cultures, D. pseudoobscura was more viable than D. subobscura, even at 23°C. Considering the rate of development, the advantage of either species depended on temperature. Thus, D. pseudoobscura developed on average, 0.25 to 2.7 days earlier than D. subobscura when kept at 23"C, while it developed later than D. subobscura at 18°C and even later at 13°C; at this temperature its mean time of development was between 3 and 9 days longer than that of D. subobscura.

In general, the results obtained in the mixed cultures at 18°C and 13°C at low densities do not show any effect of interspecific competition, since both species increased their via- bility and their rate of development when they coexisted. Nevertheless, at the highest density they were negatively affected in one or both characters. At 23°C the effect of inter-

Preadult competition between Drosophila subobscura and D . pseudoobscura 49

specific competition was clear even at lower densities. Both species underwent a reduction in developmental rate and in general the total number of flies emerging from the mixed cultures was lower than the sum of flies emerging from the respective single species cul- tures. These results could be explained under the assumption that each species exploits a different part of food or space. So, at low density, intraspecific competition is more severe than interspecific competition, since individuals of the same species have more similar needs. At the highest temperature, where larvae developed faster, and at the highest den- sity, where resources are exhaustively used, the effect of interspecific competition becomes evident, probably mainly mediated by interference.

Although D. pseudoobscura proved better adapted to higher temperatures than D. sub- obscura in the pure cultures, in the 23°C mixed cultures, D. pseudoobscura was probably subject to interference whereas D. subobscura was facilitated. In contrast, at 13"C, at which D. pseudoobscura was worst adapted, it was facilitated whereas D. subobscura was subject to interference. This suggests that the species worst adapted to a given temperature took advantage of the transformation of the environment (e. g. mechanical action on food or chemical wastes) by the larval activity of the other species to improve its own viability.

At 23°C D. pseudoobscura was clearly the dominant species since it was more viable and developed faster than D. subobscura. O n the other hand, at 18°C and especially at 13°C the outcome of competition was not so clear. At these temperatures, D. pseudoobscura was more viable than D. subobscura, but its development was slower. Developmental time is an important fitness component. In a given generation, individuals that develop faster will be able to exploit resources sooner. In addition, a faster development gives a greater rate of indrease of population size allowing a species to exploit the environment better, especially when resources are subject to seasonal changes.

Thus, it seems that at 23°C D. pseudoobscura would have advantage while at M"C, and especially at 13"C, D. subobscura may be the favoured species. This hypothesis agrees with what has already been seen in nature. In North America the ratio of D. subobsmra in rela- tion to D. pseudoobscura increases northwards (with a cooler climate). So, D. subobscura is the predominant species in the States of Washington and North of Oregon even during the summer months, while in the South of Oregon and in California D. pseudoobscura is more frequent (PREVOSTI et al. 1989). Furthermore, D. subobscura is not found South of Ojai (34'28' N) (PREVOSTI et al. 1987) while the distribution of D. pseudoobscura extends much more to the South. The distribution of D. subobscura in the Old World and in South America ends at lower latitudes than in North America. If D. subobscura has not been able to extend more to the South there, this could be due to a competitor like D. pseudoobscura or some other autochthonous species of the D. obscura group.

Acknowledgements We thank Prof. E J. AYALA for allowing us to use his laboratory and other facilities at the Univer- sity of California, during our collections in North America. We are also indebted to Prof. J. OCARA for his advice on statistical methods. This work was su ported b Grant 2844 from the Comisi6n Asesora para la Investigacih Cientifica y Tkcnica, ain, and Grant CCB-8504013 from the United States - Spain Joint Committee for the Scientilc and Technological Coopera- tion.

Zusammenfassung Konkurrenz zwischen Drosophila subobscura und Drosophila pseudoobscura wabrend

der Entwicklung

Die palaarktische Art Drosophila subobscura hat erst in letzter Zeit grofle Gebiete der Vereini ten Staaten kolonisiert, wo sie mit der endemischen Art Drosophilapseudoobscuru koexistiert. Via%%- tat und Entwicklungsrate beider Arten wurden bei 13 "C, 18 "C und 23 "C und bei einer Dichte von 10, 50, 100 und 200 Eiern pro Kulturbecher untersucht. Die beiden Arten reagierten unter- schiedlich auf die verschiedenen Dichten und Temperaturen. Sowohl die intra- als auch die inter-

50 D . J . Orengo and A. Prevosti

spezifischen Konkurrenzversuche beweisen, dai3 D . pseudoobscuru am besten an eine Temperatur von 23°C an epaflt ist, wo sie eindeutig die dominierende Art darstellt. Bei 18°C und besonders bei 13°C isttinge en die Entwicklungsrate von D. subobscuru trotz der eringeren Viabilitat kiirzer als die von b. pseudoobscuru, was der ersteren einen Vorteil verschahen konnte. Die hier wiedergegebenen Ergebnisse stimmen mit der Verbreitung der Arten in Nordamerika iiberein.

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Authors’ address: DORCAS J. ORENGO and ANTONIO PREVOSTI, Department of Genetics, Faculty of Biology, University of Barcelona, Diagonal, 645, E-08071 Bar- celona, Spain

PREVOSTI, A. ; SERRA, L.; MONCL~S, M. ; MESTRES, F.; LATORRE, A. ; % B 6 , G. ; AGUADB, M.,