Reduced metabolic rate and oxygen radicals production in stored insect sperm

Abstract

Females of internally fertilizing species can significantly extend sperm lifespan and functionality during sperm storage. The mechanisms for such delayed cellular senescence remain unknown. Here, we apply current hypotheses of cellular senescence developed for diploid cells to sperm cells, and empirically test opposing predictions on the relationship between sperm metabolic rate and oxygen radical production in an insect model, the cricket Gryllus bimaculatus. Using time-resolved microfluorimetry, we found a negative correlation between metabolic rate (proportion of protein-bound NAD[P]H) and in situ intracellular oxygen radicals production in freshly ejaculated sperm. In contrast, sperm stored by females for periods of 1 h to 26 days showed a positive correlation between metabolic rate and oxygen radicals production. At the same time, stored sperm showed a 37 per cent reduced metabolic rate, and 42 per cent reduced reactive oxygen species (ROS) production, compared with freshly ejaculated sperm. Rank differences between males in ROS production and metabolic rate observed in ejaculated sperm did not predict rank differences in stored sperm. Our method of simultaneously measuring ROS production and metabolic rate of the same sample has the advantage of providing data that are independent of sperm density and any extracellular antioxidants that are proteins. Our method also excludes effects owing to accumulated hydrogen peroxide. Our results unify aspects of competing theories of cellular ageing and suggest that reducing metabolic rate may be an important means of extending stored sperm lifespan and functionality in crickets. Our data also provide a possible explanation for why traits of ejaculates sampled from the male may be rather poor predictors of paternity in sexual selection studies and likelihood of pregnancy in reproductive medicine.

Females become infertile as the stored sperm's oxygen radicals increase

“…It is possible that sperm physiology is also very variable between species. An additional goal of this study was, therefore, to establish whether the relationship between sperm metabolism and oxygen radicals is similar to that observed in another insect species29.…”

“…extra-sperm) interference with cell metabolic pathways in order to lower the formation of oxygen radicals in the first place. This possibility has received much less theoretical714 and empirical attention29, but may be important. A previous study found sperm metabolism in female insects was reduced over several weeks of sperm storage with the corresponding intra-sperm oxygen radicals production also being low29.…”

“…This possibility has received much less theoretical714 and empirical attention29, but may be important. A previous study found sperm metabolism in female insects was reduced over several weeks of sperm storage with the corresponding intra-sperm oxygen radicals production also being low29. The reduction was proposed to be adaptive because it may delay sperm ageing and so reduce negative fitness effects on females29, but was not tested.…”

“…Here we now attempt to examine this significance. Using bedbugs, Cimex lectularius , a long-lived, sperm-storing insect30, we tested whether female infertility is lower during periods when oxygen radicals production are suppressed in sperm29 compared to times when radicals are not suppressed.…”

Ant sperm storage organs do not have phenoloxidase constitutive immune activity

“…Depending on species or lineage, sperm can be stored for days, weeks or even years making it logical to expect that spermathecae have been under selection to preserve sperm viability when that serves female fitness interests. Recent studies have indeed shown that insect females interfere with the metabolic rate of stored sperm reducing the production of harmful ROS (Reinhardt and Ribou, 2013;Ribou and Reinhardt, 2012). Another factor that affects the preservation of stored sperm is the prevalence of sexually transmitted pathogens (Knell and Webberley, 2004;Fievet et al, 2006) that might cause sperm death (Otti et al, 2013).…”