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Secondary sex characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a species.[5] In evolution, secondary sex characteristics are the product of sexual selection for traits that show fitness, giving an organism an advantage over its rivals in courtship and in aggressive interactions.[6]

The reproductive organs in male or female organisms are usually identifiable at birth and are ascribed as the Primary Somatic Sex Characteristics. In the male, this would be the penis, scrotum, and the ability to produce sperm that will help form a zygote. In the female, this would be the uterus, vagina, fallopian tubes, clitoris, cervix, and the ability to have offspring. The primary sex organs are different from the secondary sex organs because they produce gametes, which is a mature haploid germ cell male or female which will unite with another of the opposite sex during sexual reproduction to form a zygote. The secondary sex characteristics differ in that they will not be identifiable at birth, they will develop over time as the subject matures and becomes sexually active. Those characteristics are breast in females and greater muscle mass in males. Secondary sexual characteristics have an evolutionary purpose: increase the chance of breeding.[7]In the animal kingdom, an extraordinary diversity of structures exists that cannot be explained by natural selection (Darwin 1871).[8]

Ronald Fisher, the English biologist, developed a number of ideas concerning secondary characteristics in his 1930 book The Genetical Theory of Natural Selection, including the concept of Fisherian runaway which postulates that the desire for a characteristic in females combined with that characteristic in males can create a positive feedback loop or runaway where the feature becomes hugely amplified. The 1975 handicap principle extends this idea, stating that a peacock's tail, for instance, displays fitness by being a useless impediment that is very hard to fake. Another of Fisher's ideas is the sexy son hypothesis, whereby females will desire to have sons that possess the characteristic that they find sexually attractive in order to maximize the number of grandchildren they produce.[10] An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism. This idea is known as the good genes hypothesis.[citation needed]

Recently, it has been reported that glycogen metabolism is required during third larval instar for normal body size growth and even the developmental delay would not rescue the arrest of body size due to reduced glycogen levels in the larval stage35. Further, it has been evidenced that defects in glycogen metabolism are known to affect larval physiology and hence the adult fitness35. While the fat body, muscles and Central Nervous System (CNS) in larva act as the site for glycogen storage36, glycogen synthesis occurs during late larval life in fat body36,37. Furthermore, fat bodies act as a peripheral system for ecdysone metabolism38. Despite smaller larval size than controls39, the selected populations had comparable levels of glycogen (Fig. 1c, Supplementary Table S4) throughout the third instar suggesting that glycogen is the primary source of energy that is possibly driving the physiological processes leading to early expression and release of ecdysone in selected populations39 that might in turn facilitate faster development.

A developmental dietary history is known to influence adult physiology1. In general, the trade-off between longevity and life-time fecundity are well documented in Drosophila melanogaster28,41,42,43,44,45 and other holometabolous insects like Speyeria mormonia46. Consistent with dietary restriction studies, the control CS flies had lower fecundity (Fig. 2b). However, exception to larval dietary manipulation has also been reported. For example in holometabolous Lepidopteran butterfly, Speyeria mormonia, there was no independent effect of semi-starvation on realized egg laying45 suggesting an indirect effect of larval dietary restriction on fecundity. In our study, selection for faster pre-adult development that resulted in small-sized adults (Fig. 2e,f) had significantly reduced life-time fecundity (Fig. 2b, Supplementary Table S3) perhaps due to small sized ovaries18 that in turn could have affected the total realized life-time fecundity. Interestingly the life-time fecundity of the critical size flies from the selected and control populations were comparable suggesting that flies might be committing certain amount of resources to reproduction at critical time point. Further, the significantly higher fecundity of normal control flies certainly seems to be due their increased post-critical feeding period. Our results are in agreement with Min et al.2 where they reported the contribution of larval resources in early life fecundity in addition to adult diet2. However, a recent study reported reduced reproductive fitness as a consequence of small adult size due to dietary manipulation during larval growth period47. Further, in agreement with Klepsatel et al.47, our selected populations had reduced life-time fecundity owing to their small adult size perhaps due to drastic reduction in post-critical feeding duration and thus represent the cost of rapid development.

Overall, our study provides insight into the role of critical size on adult life-history in Drosophila melanogaster populations. The populations that are under simultaneous selection for faster pre-adult development and thus under curtailed food intake, perhaps commit their energy to reproduction and adult longevity immediately on attainment of critical size as indicated by comparable life-time fecundity and increased survival probability of flies that emerged from larvae fed till critical size as opposed to those that fed till natural pupation. However, the flies from the control population that emerged from larvae fed up to critical size had reduced life-time fecundity compared to those that emerged from larvae fed till natural pupation time. Taken together, the selected populations had evolved their physiology to commit available resources to adult fitness at the time of committing to irreversible process of metamorphosis. 041b061a72


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