Drosophila Littoralis Research Articles

Another way of being anisogamous in Drosophila subgenus species: giant sperm, one-to-one gamete ratio, and high zygote provisioning.

It is generally assumed that sexes in animals have arisen from a productivity versus provisioning conflict; males are those individuals producing gametes necessarily small, in excess, and individually bereft of all paternity assurance. A 1- to 2-cm sperm, 5-10 times as long as the male body, might therefore appear an evolutionary paradox. As a matter of fact, species of Drosophila of the Drosophila subgenus differ from those of other subgenera by producing exclusively sperm of that sort. We report counts of such giant costly sperm in Drosophila littoralis and Drosophila hydei females, indicating that they are offered in exceedingly small amounts, tending to a one-to-one gamete ratio after a single mating. As a result, most of them are successfully involved in a fertilization. Hence, the concept of "paternity assurance of individual sperm" arises. Evidence is further provided here that almost the entire sperm is incorporated into the egg during fertilization. Labeling with specific antibodies in fertilized eggs reveals intact axonemes up to late gastrulation. The question, then, is why selection has favored such an unusual strategy. Explanations related to some prefertilization functions are ruled out. It is therefore tentatively proposed that virtually every giant sperm constitutes a "direct paternal legacy to the embryo," which, in contrast to any male-derived nuptial gift, cannot be minimized by female remating. We suggest that dramatic shortage of giant sperm with a high prospect of fusion and increased zygote provisioning is merely another way of being anisogamous.

CRITERIA OF FEMALE MATE CHOICE IN DROSOPHILA LITTORALIS, D. MONTANA, AND D. EZOANA.

We examined sexual selection by Drosophila littoralis, D. montana, and D. ezoana females on male courtship sounds to determine whether the females use absolute or relative criteria when choosing their mates. Behavior of the females was observed, when they were courted by a single male producing normal sounds, or by a single wing-manipulated male producing abnormal sounds; and when they were courted by one or both of these males in a choice situation. The females usually accepted short-winged (but not wingless) males producing abnormal sounds, if they had no alternatives. However, if they heard the sound produced by a normal male, they rejected the deficient male. Drosophila littoralis and D. ezoana females selected between two wing-manipulated males with different wing areas. Our results suggest that the females choose their mates on the basis of relative criteria if the signals emitted by the courting males are within the range of acceptable cues.

Inheritance of male courtship sound characteristics inDrosophila littoralis

Males of Drosophila littoralis vibrate their wings during courtship to deliver a "love song." This consists of 25- to 50-ms-long pulses with a basic frequency of about 250-400 Hz, separated by 250- to 500-ms pauses. When recording the sounds of flies from several localities in Europe, we found that males of one strain from northern Finland displayed courtship sounds with an unusually low wing beat frequency (below 250 Hz). In a genetic analysis utilizing marker stocks, the anomalous frequency was found to be caused by genes on all major autosomes, the strongest factors being on the second chromosome. Interaction between genes on chromosome 2 and on the fused chromosome 3-4 was non-additive. In low-frequency sounds, the number of cycles in the pulse (CN) was decreased, so that the length of the sound pulse (PL) remained more or less unchanged. We suggest that the genetically and physiologically most thoroughly controlled trait in the sound of Drosophila littoralis is the length of the pulse.

Genetic correlation between circadian eclosion rhythm and photoperiodic diapause in Drosophila littoralis.

Populations of Drosophila littoralis are known to be latitudinally highly variable in photoperiodic adult diapause and pupal eclosion rhythm. Phenotypic correlations between the two time-measuring systems among the strains from different latitudes are, however, weak. In the present study, two differing strains were crossed reciprocally in order to search for causal (genetic) correlations between the two traits in the strains. Segregation in the F2 generations showed that variation in each trait was based on a few variable loci only. In the F2, flies having different eclosion times also differed in their diapause. This association was not complete and could have been due to genetic linkage between the traits. For that reason, the hybrid generations were raised for eight generations more to allow recombination between the traits. In F8, selection against diapause was started in the lines by raising them in a light-dark cycle of 15:9, where only females of the southern type reproduce. After eight selected generations, the lines were studied for the traits. Diapause was completely of the southern type, and the eclosion rhythm had also changed in parallel. The change in the phase of the free-running rhythm was not complete. From the present experiment, and from earlier knowledge of the geographical variation in D. littoralis, I conclude that the same pacemaker that is seen in the eclosion rhythm could also participate in daylength measurement for diapause. However, there are also noncorrelated variable parts in the measuring systems of both traits, which may mask the correlated variation.

Genetic variation in the male courtship sound of Drosophila littoralis.

Drosophila littoralis males and females emit sounds during courtship by vibrating their wings. Genetic variation in the male courtship sound of this species was studied by analyzing the sounds of males of 42 fresh isofemale strains from three localities in Finland and those of several laboratory strains originating from Europe and Caucasus. Among the fresh strains, the mean number of sound cycles in a pulse varied from 12 to 17 cycles, the length of a pulse from 39 to 51 ms, the length of a sound cycle from 2.9 to 3.6 ms, and the length of an interpulse interval (ipi) from 280 to 400 ms. The sounds of the old laboratory strains differed from each other more than the sounds of the fresh strains. There was, however, no sign of geographic differentiation.

GEOGRAPHICAL VARIATION IN THE REPRODUCTIVE CYCLE AND PHOTOPERIODIC DIAPAUSE OF DROSOPHILA PHALERATA AND D. TRANSVERSA (DROSOPHILIDAE:DIPTERA).

Adaptation to seasonal changes is of great importance to insects. The energyconsuming developmental processes are limited in the north by winter and often in the south by summer drought. Many insects spend the adverse seasons in a state of dormancy controlled by photoperiod (Beck, 1968). Geographical variation in photoperiodic reactions is well known in insects (see reviews by Danilevskii, 1965; Beck, 1968; Saunders, 1976; Hoy, 1978). In general, a clear south-north cline is observed, with northern populations entering the hibernation stage earlier than southern. Such geographical variation in Drosophila littoralis has been studied by Lumme and Oikarinen (1977) and Oikarinen and Lumme (1979) (see also Lumme, 1978). However, field studies corresponding to laboratory results have been lacking. We undertook to study the adaptation to seasonal environment by relating field observations on population dynamics to the laboratory determined photoperiodic reactions. We studied populations at different latitudes to find out how different climatic conditions are reflected in the population dynamics in the field, and, on the other hand, in photoperiodic reaction curves. Photoperiodic timing of adult reproductive diapause in the genus Drosophila was first demonstrated in D. phalerata Meigen and D. transversa Fallen (Geyspits and Simonenko, 1970). From photoperiodic reaction curves obtained in laboratory studies, Geyspits et al. (1976) inferred that D. phalerata must have a summer diapause as well as a winter one. Geyspits and Simonenko (1970) studied populations of D. phalerata from two different latitudes and found diapause differences between them. Both species are quite common and easily collected with fermenting baits. Both are known to be fungivorous (Burla and Bachli, 1968). Their distributions overlap widely, but D. phalerata reaches its ecological margin in Central Finland. It is more common in central and southern parts of Europe (Shorrocks, 1977), but its southern limit is not well known. Drosophila transversa is abundant up to the coast of the Arctic Ocean, but rather rare in Central Europe, where it is mostly found at high altitudes (Burla, 1951; Shorrocks, 1977). These species thus obviously have different adaptations to climate. Can this be seen in their phenology or photoperiodic reactions?

The environmental regulation of adult diapause in Drosophila littoralis

Drosophila littoralis overwinters in the adult stage in a reproductive diapause. During the warm season there are one or two generations in Finland. The diapause appears to be a prolongation of the post-eclosion immaturity of young females. The termination of diapause is controlled by a combination of adequate temperature and sufficiently long photophases. The diapausing status of females is ascertained by inspecting the developmental stage of their ovaries. In laboratory experiments the maturity of ovaries is not closely correlated with the receptivity of females.