DOI: 10.5061/dryad.bk3j9kdfz
title: Meiotic drive adaptive testes enlargement during early development in the stalk-eyed fly.

description: The sex ratio ‘SR’ X-linked meiotic drive system in stalk-eyed flies
 destroys Y-bearing sperm. Unlike other SR systems, drive males do not
 suffer fertility loss. They have greatly enlarged testes which compensate
 for gamete killing. We predicted that enlarged testes arise from extended
 development with resources re-allocated from the accessory glands, as
 these tend to be smaller in drive males. To test this, we tracked the
 growth of the testes and accessory glands of wild-type and drive males
 over 5–6 weeks post-eclosion before males attained sexual maturity.
 Neither of the original predictions is supported by these data. Instead,
 we found that the drive male testes were enlarged at eclosion, reflecting
 a greater allocation of resources to the testes during pupation. Testes
 grow at a higher rate during early adult development, but there was no
 evidence that this retards the growth of the accessory glands. Further
 experiments are proposed to investigate whether smaller accessory glands
 only arise in drive males post-copulation or when flies are subjected to
 nutritional stress. Our experimental findings support the idea that
 enlarged testes in drive males arise as an adaptive allocation of
 resources to traits that enhance male reproductive success.
description: Flies were
 dissected on days 0, 1, 4, 8, 12, 16, 20, 34 and 56
 (<em>N</em> = 24–53 per time point, electronic
 supplementary material, tables S1 and S2). A follow-up experiment was
 carried out with more intense measurements from day 11 to day 25 (short
 dataset), with dissections on days 11, 13, 15, 17, 19, 21, 23 and 25
 (<em>N</em> = 37–53 per time point, electronic
 supplementary material, tables S3 and S4). The thorax and eyespan of
 ice-anaesthetized flies were measured prior to dissection, using an
 Infinity Capture video microscope attached to a computer equipped with NIH
 image software (FIJI (Image J) Version 2.1.0/1.53c). The thorax was
 measured from the prothorax anterior tip, along the midline ending at the
 joint in-between the thorax and metathoracic legs [36]. Eyespan was measured from the
 outer tips of the eyes adjacent to where the stalk joins the eye
 bulb [38]. Flies were then dissected in
 15 µl of phosphate-buffered saline (PBS) using 5 mm forceps on a glass
 slide under the stereomicroscope. The testes and accessory glands were
 isolated then untangled and uncoiled without causing rupture or damage.
 Excess material such as external cuticle was removed from the slide to
 prevent distortion of the image. Another 15 µl of PBS was added before
 adding a glass cover. Images were taken using a differential interference
 contrast microscope on QCapture Pro imaging software at either x5 or x10
 magnification. The polygon selection tool in Image J was used to take area
 measurements for both the testes and accessory glands, by tracing around
 the outline of the organs. All statistical analyses were
 performed in R (v.1.4.1103). Linear regression models were used to
 identify differences in reproductive organ size between genotypes. Models
 included genotype, age (days), thorax size (body size) and relative
 eyespan. A stepwise build-up was used to add terms that improved the model
 fit. Terms that did not improve the model fit were discarded. The
 morphological traits of thorax size and relative eyespan were added as
 covariates as they are known to differ between genotypes, and correlate
 with reproductive organ size in mature adult flies [34]. Relative eyespan was calculated
 from the residuals using a linear regression model after taking into
 account thorax size, as these traits are strongly
 collinear [39]. To determine trade-offs between
 the development of the testes and accessory glands with other
 morphological traits, interaction terms were tested. Specifically,
 break-point analysis was conducted to investigate the interaction between
 genotype and age (days) on the testes as a proxy of growth rate. Mean and
 standard error trait sizes (mm) are reported throughout. See electronic
 supplementary material, for all models.
description: Microsoft Excel