DOI: 10.5061/dryad.zcrjdfndh
title: Social polyandry shapes sperm morphology

description: Sexual selection is a major driver of trait variation, and the intensity
 of male competition for mating opportunities has been linked with sperm
 size across diverse taxa. Mating competition among females may also shape
 the evolution of sperm traits, but the interplay between female-female
 competition and male-male competition on sperm morphology is not well
 understood. We evaluated variation in sperm morphology in two species with
 socially polyandrous mating systems, in which females compete to mate with
 multiple males. Northern jacanas (Jacana spinosa) and wattled jacanas (J.
 jacana) vary in their degree of polyandry and sexual dimorphism,
 suggesting species differences in the intensity of sexual selection. We
 compared mean and variance in sperm head, midpiece, and tail length
 between species and breeding stages, because these measures have been
 associated with the intensity of sperm competition. We found that the
 species with greater polyandry, northern jacana, has sperm with longer
 midpieces and tails, as well as marginally lower intra-ejaculate variation
 in tail length. Intra-ejaculate variation was also significantly lower in
 copulating males than in incubating males, suggesting flexibility in sperm
 production as males cycle between breeding stages. Our results indicate
 that stronger female-female competition for mating opportunities may also
 shape more intense male-male competition by selecting for longer and less
 variable sperm traits. These findings extend frameworks developed in
 socially monogamous species to reveal that sperm competition may be an
 important evolutionary force layered atop female-female competition for
 mates.
description: We collected samples from a population of northern
 jacanas (<em>Jacana spinosa</em>) in La Barqueta, Chiriqui
 (8.207N, 82.579W) and a population of wattled jacanas (<em>J.
 jacana</em>) near Chepo (9.166N, 79.122W) from 16 May to 1 July
 2018. We sampled 11 northern jacana males (5 copulating, 7 incubating) and
 7 wattled jacana males (4 copulating, 3 incubating). Ductus deferens were
 preserved in 5% formaldehyde solution in PBS in the field. We also
 measured testes mass and volume, and body mass.  We
 prepared sperm samples for visualization with brightfield
 microscopy. Using a dissection microscope at 10x magnification, we used
 forceps to make an incision in the seminal glomus and squeezed out the
 fixed ejaculate onto a microscope slide in 15-20 μL of 5% formaldehyde. We
 then pipetted the ejaculate-formaldehyde mixture into a microcentrifuge
 tube, mixed it with a pipette, and pipetted it onto a fresh slide with a
 slipcover. We photographed samples immediately after slide preparation to
 avoid crystallization of the buffer. We identified at least 10 intact
 sperm cells per male and photographed them using an EVOS FL fluorescence
 microscope at 600x magnification. To compare sperm
 morphology between the two species, we focused on the length of three
 sperm traits: tail, midpiece, and head length. We measured the length of
 each trait using the segment line tool in ImageJ v1.8. We measured the
 tail based on the end of the midpiece to terminal end of the flagellum. We
 distinguished the head from the midpiece by a change in transparency and
 tapering shape, based on guidelines for Charadriiform sperm morphology
 (Jamieson 2007).
description: Please see ReadMe file.