Results
Sub-lethal
temperature effects on behaviour and male reproductive output
On the basis that males of temperate Drosophila melanogasterstrains become sterile at 30°C (Chakir et al., 2002; Petavy et al.,
2001), we investigated the recovery dynamics for males of a temperate
Portuguese D. melanogaster strain (Martins et al., 2014)
developed at mid (29°C) or sub-lethal (31°C) temperatures. First, we
documented viability of larvae under the different developmental
temperatures and found those to be clearly challenging as only 45.5%
respective 23.7% of flies eclosed when exposed to 29°C or to 31°C, in
contrast to 95% of control flies eclosed successfully. Remaining at 29
or 31°C was stressful to adults as well, as males clearly did not
recover fertility in contrast to males allowed to recover (Fig. 1A, C
and table 1).
We next assessed the fecundity of heat-challenged males and found
females mated once to a heat-challenged male laid significantly fewer
eggs compared to females mated to control males (Fig. 1B, table 1).
Males kept at the challenging temperature after eclosion induced lower
numbers of eggs laid throughout, while a small improvement was seen for
males allowed to recover. We independently repeated this experiment with
the addition of sampling also day 1 after eclosion and found the pattern
to be robust (Supplementary Fig. S1A, B and Supplementary table S2).
The offspring counts revealed that recovering males were initially
sterile and recovered at different rates (see Fig. 1C, interaction term
table 1), with offspring numbers approaching those of control males on
day 6. This is matched by a steep increase in egg-to-adult survival
between day 2 and 4 (see Fig. 1D), while males remaining at the
challenging conditions produced very few offspring and had a low egg to
adult survival (Fig. 1D, table 1). No adult flies were produced by males
grown and kept at 31°C. Males grown and kept at 29°C produced on average
82% fewer offspring on day 6, with respect to control males, while
those allowed to recover after eclosion had 36% fewer offspring. The
slight increase for males recovering from development at 31°C still
resulted in a mean reduction of 63% in offspring produced. Hence, at
this temperature males suffer a severe fitness loss even if allowed to
recover, while persistent heat stress results in near complete sterility
even at the moderate challenge.
When considering the mating behavior for individual pairs, we overall
found little effect of a moderate heat-challenge on mating success and
only males experiencing a severe heat-challenge were negatively
impaired. Males raised at 31°C gained few copulations compared to males
from the other treatment groups (χ2 = 66.135,df = 4, P < 0.0001; Fig. 2A). Males raised at
31°C had longer mating latencies (GLM with gamma distribution:Deviance = 7.582, F = 3.002, df = 4, P =
0.021; Fig. 2B) while there was no effect on copulation duration (GLM
with poisson distribution: Deviance = 8.867, df = 4,P = 0.065; Supplementary Fig. S2A).
In addition to single mating productivity, we also tested male sperm
competitive ability after developmental heat-exposure. We document a
severe negative impact of heat on male sperm defense ability (GLM with a
quasibinomial distribution: Deviance = 5173.8, F = 22.177,df = 4, P < 0.0001, Fig 2D), even after we
allowed males to recover for 5 days.
Additionally, heat-challenged males were unable to prevent female
remating, regardless of the possibility for recovery, as all females
remated when the second male was present, while only 35.1% of females
first mated to a control male remated (χ2 =
82.624; df = 4, P < 0.0001). This pattern was
also reflected in second mating latencies (GLM with quasibinomial error
distribution; Deviance = 73.482, F = 181.55, df =
1, P < 0.0001; see Fig. 2C).