Low but genetically variable male mating ability in a tropical Drosophila despite fitness benefits of remating

Abstract

Mating success is the main source of fitness variation in males, meaning that males should capitalise on all opportunities for mating. Strong selection on male mating success should also reduce genetic variation in male mating traits relative to other traits. We quantified mating latency, mating duration and productivity in males of the tropical fruitfly, Drosophila birchii, from 30 isofemale lines collected from across two elevational gradients, when they were given opportunities to mate with up to four females consecutively. Male remating rates were low compared to other Drosophila (only 14 - 27% of males achieved a fourth mating), with mean mating durations approximately doubling across successive copulations. However, although successive remating produced progressively fewer offspring, it consistently increased overall male reproductive success, with males that mated four times more than doubling offspring number compared to males mating only once. We also found no reduction in the productivity of sons emerging from later matings, indicating a sustained cumulative fitness benefit to remating. Heritable variation was observed for most traits (H2 = 0.035 - 0.292) except mating latency, but there was no divergence in trait means with elevation. The observed restricted remating ability of male D. birchii, despite the clear benefits of remating, may be due to a low encounter rate with females in the field, leading to high investment per gamete (or ejaculate). However, it remains unclear why genetic variation in these traits is high, given we observe no variation in these traits across elevational gradients known to affect local population density.