The drive to reproduce has led to many of the flashiest traits observed in nature. For instance, male deer use antlers to fight one another for access to female mates, while male peacocks display their extravagant tails to impress peahens. These are examples of sexually selected traits, or traits that increase reproductive success. Sexually selected traits evolve because they increase “fitness”. In other words, specific traits cause organisms to produce more offspring in the next generation–these offspring inherit the sexually selected traits from their parents, causing the traits to continue rising in frequency with each subsequent generation. While the traits themselves are often flashy, perhaps the most extreme example of a sexually selected trait comes from an inconspicuous source. Drosophila, a class of insects colloquially referred to as fruit flies, appear unassuming on the outside, but produce the largest sperm in the animal kingdom. For context, human sperm is roughly 60 micrometers long, which is close to 1000th of a centimeter (obviously, a very small fraction of a human’s body size). Drosophila melanogaster, the fly species famous for its frequent use in biological research, has roughly 2,000 micrometer long sperm—1/5 of a centimeter, and approximately half the fly’s body size. Finally, and most incredibly, the species Drosophila bifurca produces the longest sperm cells of any known organism at nearly 6 centimeters long—over 20 times the length of their body!
There are two forces driving the evolution of such extreme sperm size. The first is male-male competition—like how deer use antlers, Drosophila males use sperm to compete with one another. Most female Drosophila mate frequently, with different males, which means that each male’s sperm is competing with every other male to fertilize her eggs. Drosophila females also have the amazing ability to store sperm for days or weeks after mating, and they do so primarily in a reproductive organ called the seminal receptacle (see image 2 below). When a Drosophila female mates with a new male, his sperm will attempt to dislodge and replace sperm from the previous male. Longer sperm are better able to displace previous males’ sperm, and this evolutionary advantage enables males with the longest sperm to fertilize most of the available eggs (high fitness!). Thus, males with long sperm produce many offspring that also have long sperm in the next generation. Repetition of this pattern over many, many generations enabled sperm length to increase to the astounding size we see today.
However, this isn’t the complete story. Much like a peahen that has evolved to mate with particularly elaborate males, Drosophila females have evolved to fertilize their eggs with particularly long sperm. Only large, healthy males are able to produce large quantities of gigantic sperm, and if a female reproduces with healthy males, her offspring will have genes that make them healthy, too. Females can therefore pass on more genes to subsequent generations (i.e., increase their fitness) by fertilizing their eggs with sperm from only the highest quality mates. While peahens accomplish this goal by evolving to mate with peacocks with elaborate tails, Drosophila females have accomplished this by increasing the length of their seminal receptacle. When the seminal receptacle is long, it becomes absolutely vital for males to have long sperm; if it isn’t long enough, a male’s sperm will be largely replaced by the next male she mates with. Females with long seminal receptacles therefore fertilize their eggs with only the longest sperm they can find. It is likely that female seminal receptacles increased in length first, due to the increase in female fitness, and male sperm length followed suit (Miller and Pitnick, 2002); this coevolution continued over many generations, leading to the tremendous seminal receptacle and sperm length that we observe now in Drosophila bifurca.
While the species Drosophila bifurca has particularly notable reproductive morphology, it’s only the tip of the iceberg in terms of reproductive diversity in Drosophila. In fact, increasing seminal receptacle length is only one way that Drosophila females influence the paternity of their offspring. Drosophila pseudoobscura females, for instance, may have evolved to cut off matings with undesirable males prematurely in order to reduce fertilization of his sperm (Peckenpaugh et al. 2021). As a graduate student in the Moyle lab at Indiana University, I am further characterizing reproductive diversity across 15 Drosophila species, spanning the breadth of ~60 million years of evolution. Specifically, I am determining how frequently females of different species of Drosophila remate (i.e., does this happen multiple times per day? Or once in their lifetime?). This work will help to determine how many opportunities females have to influence which males’ sperm fertilize their eggs. I am also measuring behavioral traits (e.g., mating duration) and morphological traits (related to form or structure–e.g., sperm length, male and female reproductive tract dimensions, pigmentation of male and female reproductive organs; see image 3 below). The broad diversity in these traits across species allows me to ask questions such as: in species where females remate frequently, and they therefore have many opportunities to bias sperm use in favor of preferred mates, are their seminal receptacles longer? Or is mating duration shorter? Perhaps males have responded to the increased sexual selection pressure by investing in larger testes, or brighter testes (which females can then see through their abdomen)? By comparing across many species, I hope to identify general patterns in how these sexual traits have evolved. Despite the many unanswered questions still left to explore, this data should bring us one step closer to understanding what drives the evolution of this exciting diversity.
Miller, G. T., & Pitnick, S. (2002). Sperm-female coevolution in Drosophila. Science, 298(5596), 1230-1233. https://doi.org/10.1126/science.1076968
Peckenpaugh, B., Castillo, D. M., & Moyle, L. C. (2021). Testing potential mechanisms of conspecific sperm precedence in Drosophila pseudoobscura. Journal of Evolutionary Biology, 34(12), 1970-1980. https://doi.org/10.1111/jeb.13946