Abstract
Due to its influence on body size, timing of maturation is an important life-history trait in ectotherms with indeterminate growth. Comparison of patterns of growth and maturation within and between two populations (giant vs. normal sized) of nine-spined sticklebacks (Pungitius pungitius) in a breeding experiment revealed that the difference in mean adult body size between the populations is caused by differences in timing of maturation, and not by differential growth rates. The fish in small-sized population matured earlier than those from large-sized population, and maturation was accompanied by a reduction in growth rate in the small-sized population. Males matured earlier and at smaller size than females, and the fish that were immature at the end of the experiment were larger than those that had already matured. Throughout the experimental period, body size in both populations was heritable (h2 = 0.10–0.64), as was the timing of maturation in the small-sized population (h2 = 0.13–0.16). There was a significant positive genetic correlation between body size and timing of maturation at 140 DAH, but not earlier (at 80 or 110 DAH). Comparison of observed body size divergence between the populations revealed that Q ST exceeded F ST at older ages, indicating adaptive basis for the observed divergence. Hence, the results suggest that the body size differences within and between populations reflect heritable genetic differences in the timing of maturation, and that the observed body size divergence is adaptive.
Highlights
In many organisms, and in particular in ectotherms with indeterminate growth, age and size at maturation are among the most important life-history traits affecting fitness [1,2,3]
Growth and timing of sexual maturation The patterns of growth differed between the two populations: while the mean body size of Pyorealampi individuals increased more or less linearly throughout the observation period, the growth of the Helsinki fish began to slow down at 80 days after hatching (DAH; Fig. 1a)
Pyorealampi fish were significantly smaller than Helsinki fish at 20 DAH, whereas at 50 and 80 DAHs no significant difference could be detected (50 DAH: likelihood ratio test (LRT) = 1.9, P = 0.166; 80 DAH: LRT = 0.45, P = 0.504)
Summary
In particular in ectotherms with indeterminate growth, age and size at maturation are among the most important life-history traits affecting fitness [1,2,3]. In environments with high mortality rates, such as in populations where individuals are subject to intensive predation, early maturation at smaller size is expected to evolve as compared to populations with lower mortality risks [1,2,7] Both the proximate determinants of timing of maturation and somatic growth rate are known to be influenced by environmental and genetic effects Little is known about the genetics of maturation and its role in determining final body size (but see: [25,26,27]) It is debatable whether maturation can evolve independently of growth or whether the timing of maturation is linked to growth preceding maturation The probabilistic maturation reaction norm approach, which is often used in the analyses of fisheries-induced evolution, relies on the assumption that variation in growth is largely environmental and that genetic changes in maturation are seen after controlling for changes in growth (e.g. [27,28])
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