Direct visualization of sperm competition and sperm storage in Drosophila

Abstract

Drosophila females engage in multiple matings [1] [2] [3] [4] even though they can store sperm in specialized organs for most of their life [5]. The existence of sperm competition in Drosophila has been inferred from the proportion of progeny sired by the second male in double-mating experiments [6] [7] [8]. Investigators have used this approach to quantify genetic variation underlying sperm competition [8] [9] [10], to elucidate its genetic basis [11], to identify the dependence of different male competitive ability on the genotype of the females with which they mate [12] and to discern the potential role of sperm competition in species isolation [13] [14]. This approach assumes that the sperm from two males stored in a female compete to fertilize the eggs. The mechanism by which sperm competition is accomplished is still unknown, however. Here, fluorescence microscopy, cytometry, and differently labeled sperm were used to analyze the fate of sperm inside the female's sperm storage organs, to quantify sperm competition, and to assess how closely paternity success corresponds to the appearance and location of the sperm. The results show that the first male's sperm is retained for a shortened period if the female remates, and that the second males that sire more progeny either induce females to store and use more of their sperm or strongly displace resident sperm.

Sperm death and dumping in Drosophila

“…1; Fisher's PLSD, critical difference ¼ 0.16, difference ¼ 0.17, P ¼ 0.04) because remated females received new viable sperm from the copulating male. Therefore, contrary to previous assumptions [7][8][9] , incoming seminal fluid does not cause resident sperm death and, instead, sperm physiology is influenced by sperm storage rather than seminal fluid. Previous research has found that 'incapacitation' is more effective on older resident sperm 8 , but how males could incapacitate rival sperm without damaging their own is unknown, especially because there is no self-sperm recognition 10 .…”

“…In addition to generating competition between the ejaculates of different males 1,2 , multiple mating may allow females to bias sperm use 3,4 .I nDrosophila melanogaster, the last male to inseminate a female sires approximately 80% of subsequent progeny 5 . Both sperm displacement, where resident sperm are removed from storage by the incoming ejaculate of the copulating male 6 , and sperm incapacitation, where incoming seminal fluids supposedly interfere with resident sperm 7 , have been implicated in this pattern of sperm use [5][6][7][8][9][10][11][12] . But the idea of incapacitation is problematic because there are no known mechanisms by which an individual could damage rival sperm and not their own.…”

“…The physiological nature of incapacitation is unknown, but it is assumed to include either sperm inviability or sperm death 16,17 . The only data suggestive of sperm incapacitation come from experiments in D. melanogaster [7][8][9] , one of the best-studied systems of sperm competition. In this species, males transfer sperm and a large number of seminal peptides (Acps) to females during copulation.…”

“…In this species, males transfer sperm and a large number of seminal peptides (Acps) to females during copulation. These male-derived Acps have been implicated in sperm incapacitation and are therefore invoked to explain patterns of paternity 1,[7][8][9] . In addition, although not observed directly, rival sperm have been suggested to physically displace resident sperm from storage 1,8,10,11 , putatively explaining sperm-precedence patterns.…”

“…Twenty-four hours later, we dissected females to assess sperm viability by counting the number of live compared with dead sperm in the seminal receptacle, as this organ is believed to be the primary source of sperm for fertilization and it is only this organ in which sperm incapacitation is thought to occur 8,9 . We found a strong treatment effect (F 2,89 ¼ 4.89; P ¼ 0.009) indicating that the Figure 1 Effects of copulation on sperm death.…”