Seasonal time constraints reduce genetic variation in life‐history traits along a latitudinal gradient

Abstract

Time constraints cause strong selection on life-history traits, because populations need to complete their life cycles within a shorter time. We therefore expect lower genetic variation in these traits in high- than in low-latitude populations, since the former are more time-constrained. The aim was to estimate life-history traits and their genetic variation in an obligately univoltine damselfly along a latitudinal gradient of 2730 km. Populations were grown in the laboratory at temperatures and photoperiods simulating those at their place of origin. In a complementary experiment, individuals from the same families were grown in constant temperature and photoperiod that mimicked average conditions across the latitude. Development time and size was faster and smaller, respectively, and growth rate was higher at northern latitudes. Additive genetic variance was very low for life-history traits, and estimates for egg development time and larval growth rate showed significant decreases towards northern latitudes. The expression of genetic effects in life-history traits differed considerably when individuals were grown in constant rather than simulated and naturally variable conditions. Our results support strong selection by time constraints. They also highlight the importance of growing organisms in their native environment for correct estimates of genetic variance at their place of origin. Our results also suggest that the evolutionary potential of life-history traits is very low at northern compared to southern latitudes, but that changes in climate could alter this pattern.

Cannibalism and activity rate in larval damselflies increase along a latitudinal gradient as a consequence of time constraints

“…The populations were situated in southern France (43°N), Poland (54°N) and northern Sweden (66°N). Estimates of population densities at these populations are given in Sniegula et al [30], and in general the densities are lower in the north. The distances between the replicate populations ranged from 5 to 151 km and they were all permanent waters.…”

“…The temperatures that the eggs and larvae experienced during both experiments were derived from the FLake lake model [33]. For a detailed description of the FLake model, see Sniegula et al [30]. All applicable institutional and/or national guidelines for the care and use of animals were followed.…”

“…Upon arrival at the laboratory, we placed the eggs in separate plastic containers (12 × 8 cm, height 5 cm) filled with 250 ml of dechlorinated tap water and put them in a climate chamber with temperature 21.9 °C and photoperiod L-D 19:25–4:35. We set this temperature because individuals from each latitude experience it for at least several hours a day during the main growing season in nature [30]. The photoperiod reflected the longest day length at the mid-latitude across our sampled latitudes, 55°N.…”

“…Thereafter the eggs were transferred to a climate chamber with a constant temperature of 21.9 °C and L-D 19:25–4:35. Importantly, the L-D 19:25–4:35 triggered the eggs from the central and southern latitudes to hatch very synchronously and the northern latitude eggs to hatch synchronously enough for there to be a sufficient number of individuals for the cannibalism experiment [30]. Note that due to different sampling dates (see above), the cannibalism experiment started at different days for each latitude.…”

“…Indeed, many studies have shown that northern populations do have a higher growth and faster development than southern populations, e.g. [27–30], but no studies have examined how cannibalism varies along a latitudinal gradient.…”