Marlina Montano

marlina98@hotmail.com

For any species to be ‘successful’ on this planet, a sufficient means of reproduction must be found. For many animal and plant species, asexual reproduction is adequate. Asexual reproduction being the easiest method for plants and animals. However, for many eucaryotic animals and plants much larger and more complex, including Drosophila Melanogaster, sexual reproduction is much more fitting for the large amounts of genetic information that must occur. Unfortunately, with sexual reproduction as the primary means of DNA exchange and recombination, many consequences can occur: unforeseen, expected, negative or positive. Negative effects mostly being the rejection of a male by a female. I will attempt to review some of these consequential ‘problems’ with Drosophila melanogaster mating and review the behaviors that develop from them. Although it is impossible to anticipate all of natures methods for nullifying copulation, I will bring forth such issues as size (in males), roles in male and female behavior, egg deposition, dietary necessity, costs of receiving sperm (in females), copulation duration and problems associated with male accessory gland products and how they are used in the sexual behavior of D. melanogaster. Each of these issues may or may not have subsequent consequences in the reproductive fitness of the species Drosophila Melanogaster. Some of the negative effects can include non-fertilization of females, loss of longevity, a decline in mating speed, and small production of progeny. Positive effects can include high progeny production, low remating frequency, and destruction of sperm from previous mates.

Mating behavior in Drosophila melanogaster starts with the male orienting itself toward and following a female, a behavior often referred to as tracking. After tracking, the male approaches the female to tap her abdomen, with his forelegs, followed by unilateral wing vibrations. The promotion of copulation appears to result from reduced locomotion by the females after exposure to the song. When the courting male is not rejected by the female, he turns to the back of the female and licks her genitalia with his probiscus. After licking, the male attempts to copulate, mounting the female’s back while gripping the female’s raised wings with his forelegs. When the female is sufficiently receptive, she opens the vaginal plate, allowing the male to copulate. After stable copulation for 15-20 minutes, the male dismounts the female after genital uncoupling. (Yamamoto D 1999) Sexual behavior in Drosophila melanogaster can inhibit the initial attractiveness of a mate, prevent insemination and limit the population size. These consequences stemmed from behaviors that are genetically and environmentally induced. The following compilation will give an elementary overview of the consequences of certain sexual behaviors , correlating with the steps of copulation (courtship, copulation, egg laying), and some complications involved.

The tracking behavior is apparently guided visually, because defects in vision tend to misorient in relation to the female, resulting in the termination of tracking, and consequently the whole sequence of courtship.(Yamamoto D 1999) Vision is extremely important for mating preferences in D. melanogaster. D. melanogaster males discriminate strongly against females which have ectopic male-specific pigmentation but are otherwise normal. Males which are visually impaired mate equally with male-specific pigmented females and normal females. (Kopp A 2000) Therefore, it appears that female pigmentation is important in determining their attractiveness to males, but male pigmentation appears to have little or no effect on female mating preferences. However, it must be noted that pigmentation discrimination occurs to females that appear to have the pigmentation of a male and not necessarily to females whose pigmentation is phenotypically female. The absence of male-specific pigmentation in females may be maintained by sexual selection (Yamamoto D 1999). The role of tapping in courtship has not been evaluated critically, however, it is speculated that the male can ‘taste’ the body surface of the female by tapping it with his forelegs (which bear gustatory as well as mechanosensory bristles). Males that have an enhanced tendency to male-to-male courtship have fewer gustatory bristles than those of males that court females. Feminization of parts of the antennal lobe and mushroom body by targeted expression of a female-determining gene drives the male to court other males. The gustatory sensation of pheromones obtained during tapping by a normal male helps him to continue courting. D. melanogaster pheromone 5-T tends to delay male initiation of courtship with another male, serving as an olfactory cue for distinguishing males from females. (Yamamoto D 1999). The male will then begin to vibrate his wings in a celebrated ‘courtship song’. The impact of wing vibration in males upon females is primarily auditory, although the fluttering of the wings also accelerates dispersal of hydrocarbons from the body surface, thereby potentially facilitating olfactory communication between the male and female (Yamamoto D 1999). Unilateral wing vibration is regarded as a hallmark of male courtship in D. melanogaster and has been extensively studied behaviorally, physiologically and genetically. Unilateral wing vibrations produce two distinct types of sound, the ‘pulse song’ and the ‘sine song’, collectively called the courtship song. The pulse song is composed of a series of single-peaked pulses with a mean interpulse interval (IPI) of about 35 ms. The IPI fluctuates regularly with a mean cycle of 55 s (Yamamoto D 1999). Virgin melanogaster females exposed to a pulse song had an increase in receptiveness. The sine song consists of a modified sine wave of 160 Hz and is effective in slowing down the female’s movement (Yamamoto D 1999). The pulse song enhances male locomotor activity and stimulates other males to begin to sing. At this point in the process of courtship the female can decide to accept the male as a mate or reject him. the rejection signal is another type of auditory signal exchanged between male and female courting flies. The sound is generated during bilateral wing flicking my males and unreceptive young females (Yamamoto D 1999). When the courting male is not hampered by the rejection signal, he turns to the back of the female and licks her genitalia with his probiscus. It is suggested that the licking by the male is triggered by extrusion by the female of the ovipositor, which the male contacts with his probiscus. In D. melanogaster, extrusion was considered to be an expression of the female’s unwillingness to copulate. However, extrusion may simply be a posture of females to emit chemical substances. (Yamamoto D 1999). It is thought that these chemicals can be either inhibitory or excitatory. Extrusion per se does not encourage or discourage males from courting females, but chemical substances emitted during the extrusion do alter the subsequent behavior of the male (Yamamoto D 1999). For example, if a female extrudes eggs, then the male immediately terminates his attempt at courtship. Virgin females repeatedly extrude with no inhibitory affect, as males continue to court vigorously. Up to this point, courtship has been fairly complicated and behaviors by both sexes are codependent. It is with costs that courtship is even physically possible.

Exposure of male D. melanogaster to virgin females increases the rate of courtship, mating and production of sperm and accessory fluid (Cordts R 1996). Experimental males were induced to perform only parts of sexual activity, Comparison of longevities between these males showed that courtship alone was sufficient to reduce male life span. In D. melanogaster reproductive activity reduces longevity and future fertility of both males and females (Cordts R 2000). This is probably because males will have to court many females before actually mating.

Females of most animal species are usually inseminated by more than one male, which allows sperm from different males to compete for fertilization. To prevent invasion of sperm from other males, Drosophila males elicit a rejection behavior in their mates after copulation. Sperm competition is a common phenomenon among most animal species and is a powerful evolutionary force that influences both behavior and physiology. In Drosophila, the so-called second or last-male sperm fathers most of the female’s offspring. This sperm precedence is explained by two mechanisms: the sperm of earlier matings is either displaced by the fresh sperm or incapacitated by the seminal fluid produced by the accessory glands of the last male. However, earlier mating counteract this effect in females by drastically enhancing egg laying (oviposition) and repressing sexual receptivity, i.e., the acceptance of males for remating, and thus maximize the number of eggs fertilized by the sperm before the next mating. (Xue L 1999) Both behavioral responses can persist for 9-11 days after a single mating event and require, in addition to sperm, the transfer during copulation of components secreted by the male accessory glands. (Xue L 1999) Males which secrete seminal fluid but no sperm, induce in their mates a transient increase in oviposition and a decrease in receptivity for only 1 day (Xue L 1999). This is evidenced by experiments in which females have been mated with males who don’t produce either sperm or seminal fluid, and have no reaction. Therefore it has been proposed that both responses are composed of two phases: a short-term phase lasting for 1 day and a long-term phase persisting for an additional 8-10 days. The short-term phase is thought to be activated only by seminal fluid and the long-term phase, only by sperm. (Xue L 1999) According to this model, females who have been mated with males which produce only sperm and not seminal fluid should exhibit a long-term phase reaction for oviposition. In an experiment done to determine the individual contributions of the seminal fluid and sperm to female behavior, three types of males were used as mates: normal males (which produce both sperm and seminal fluid), males that only produce sperm (S males), and males that only produce seminal fluid (SF males). Virgin females deposit only about 4 eggs per day, females lay an average of 43 eggs on the first day after a single mating with a normal male (Xue L 1999). The oviposition of these females is enhanced to 67 eggs on the second day and remains elevated, albeit at slightly lower levels, for at least 5 days after mating. Females mated with SF males show a partial stimulation of oviposition during the first day after mating and lay an average of 25 eggs. Oviposition induced by SF males declines during the second day and virtually reached virgin levels after only 3 days. Females mated with S males lay an average of 11 eggs on the first day and increases slightly for the next 5 days. Sperm without accessory gland secretions is able to elicit only a third of the long-term response in normal male oviposition. Therefore, the remaining two-thirds must be caused by accessory gland secretions. (Xue L 1999)

Female receptivity can also be affected by sperm and seminal fluid. Mature virgin females mate readily, but reject subsequent copulation attempts during the first 3 days after mating until the original receptivity is restored gradually after another 7-8 days. Mates of S males accept further copulation at a rate indistinguishable from that of virgin females at all tested time points. Suggesting that sperm cannot elicit any rejection behavior in females in the absence of seminal fluid. Females mated to SF males display nearly complete rejection 12 hours after copulation. Receptivity is recovered in at most 2 days. This portion of the experiment indicates that rejection behavior is triggered exclusively by accessory gland secretions and that its persistence depends on the presence of sperm, which in turn may stabilize the activity of accessory gland fluid (seminal fluid). Sperm may also stabilize the unknown factors that diminish the female sexual attractiveness. (Xue L 1999) For males, mated females become less attractive than virgin females. Although the drop in female attractiveness is not triggered by the receipt of sperm or seminal fluid, its persistence depends on the presence of sperm. This behavior is only advantageous to males that have had the opportunity to mate.

Post-copulatory behaviors

Female Drosophila melanogaster generally lay their eggs on fermented media. And in the wild, the main food source is represented by fermented fruit. This is done to provide nourishment for her newly laid clutch of offspring. (Cadieu N 2000) The offspring will only take a few days to eclose, and then only a few more hours more for the offspring to become sexually capable adult D. melanogaster. The repetitive cycle will begin with each newly eclosed clutch of flies.

One must keep in mind that selection on age at reproduction can have a large factor on the lifeline of the newly eclosed clutch. Its is shown that as a female gets older, she mates at a higher frequency than that of virgin females. Males show an increase in mortality when exposed to females and females also show a cost of reproduction resulting in increases in mortality. (Sgro C 2000) It is also known that in the presence of actively mating flies, virgin D. melanogaster males and females become more efficient at obtaining matings (Joshi A 1999). These additional behaviors have effects, even in the most minute of ways, on sexual reproduction and fitness of Drosophila melanogaster.

Behaviors controlling the propensity to mate or remate can have large effects on fitness, because the decision to mate during a given period can have direct effects on current and future reproduction and hence fitness (Sgro C 2000). There are many opportunities in which males and females can express their desire or lack of desire to mate, and each one of these complex behaviors is capable of becoming nullified by even the slightest genetic variation. Drosophila melanogaster has many intricate and complex behaviors which I cannot attempt to understand or explain. This very general and nonspecific overview has served its informative purpose if a general understanding of some behaviors has ensued, but has no actual experimental findings.