Sperm Biology: Size Indeed Matters

Although sperm are typically thought to be small, tadpole-like cells produced in vast quantities, sperm size, form and number all vary tremendously across species. Sperm form ranges from amoeboid or discoid to multiflagellate elongated structures [1]. Even within the more familiar uniflagellate spermatozoa, length varies by several orders of magnitude, from the tiny 28μm long porcupine sperm to the giant sperm of Drosophila bifurca, a whopping 5.8 cm long, 20 times larger than the fly! (Figure 1) [2–4]. To add to this bewildering array, not all sperm morphs can fertilize an egg [5], and sperm length varies considerably within a species [5,6]. While the selective forces responsible for this perplexing diversity have long been sought, unequivocal general explanations for sperm length variation remain elusive. Nevertheless, the major force behind the evolution of sperm size and number is thought to be sperm competition, as first proposed by Parker in 1970 (reviewed in [7]). Sperm competition occurs whenever the ejaculates of two or more males compete to fertilize a female’s ova. Two new studies [8,9] have now used methods of experimental evolution, coupled with sperm competition experiments, to gain novel insights into the adaptive significance of sperm length. Sperm size variation has been extensively studied in Drosophila [4,5,8,10], in part because sperm length variability is as great in this taxon as it is across the rest of the animal kingdom. The available evidence strongly suggests that female preference has selected for the gigantic sperm often seen in this group. This conclusion was cemented by a study [8] in which D. melanogaster was subject to bidirectional selection on sperm length and the length of the female sperm storage organ (seminal receptacle), and then fertilization success was assessed in a series of sperm competition experiments. In the study by Miller and Pitnick [8], females that varied in receptacle length were each mated to two males that differed in sperm length. In short-receptacled females, all males had equal fertilization success, regardless of sperm length. As the receptacle length increased, however, so did the fertilization advantage to males with longer sperm. Consistent with this experimental outcome, laboratory populations evolving longer female receptacles also evolved longer sperm as a correlated response. In contrast, sperm length did not change over time in populations evolving shorter receptacles. Interestingly, it appears that the fertilization success of long and short sperm males in shortreceptacled females becomes more uniform as traits increasingly diverge. This suggests that once sperm exceeds some threshold length relative to the receptacle, female preference fades. Miller and Pitnick [8] conclude that giant sperm in Drosophila are best seen as analogs of the peacock’s tail, evolving through female choice. This finding may partly explain the frequently observed association between sperm length and the size of female sperm storage organs across species [11,12]. As an aside, it is also interesting that the fertilization success of second males is extremely variable across replicate experiments relative to the small variation within trials. Why this occurs is unclear. But what about species other than Drosophila? As reported in this issue of Current Biology, Gage and Morrow [9] evaluated the importance of sperm length and number as determinants of fertilization success in the cricket Gryllus bimaculatus. As in the Drosophila study [8], crickets had been subject to divergent selection on sperm length, and males were placed into competition by sequentially mating pairs of males to a Dispatch Current Biology, Vol. 13, R355–R356, April 29, 2003, ©2003 Elsevier Science Ltd. All rights reserved. DOI 10.1016/S0960-9822(03)00275-6