Abstract
Mounting an immune response to fight disease is costly for an organism and can reduce investment in another life-history trait, such as reproduction. The terminal investment hypothesis predicts that an organism will increase reproductive effort when threatened by disease. The reproductive fitness of amphibians infected with the deadly fungal pathogen Batrachochytrium dendrobatidis (Bd) is largely unknown. In this study, we explored gametogenesis in two endangered and susceptible frog species, Pseudophryne corroboree and Litoria verreauxii alpina. Gametogenesis, both oogenesis and spermatogenesis, increased when animals were experimentally infected with Bd. In P. corroboree, infected males have thicker germinal epithelium, and a larger proportion of spermatocytes. In L. v. alpina, infected males had more spermatic cell bundles in total, and a larger proportion of spermatozoa bundles. In female L. v. alpina, ovaries and oviducts were larger in infected animals, and there were more cells present within the ovaries. Terminal investment has consequences for the evolution of disease resistance in declining species. If infected animals are increasing reproductive efforts and producing more offspring before succumbing to disease, it is possible that population-level selection for disease resistance will be minimized.
Highlights
Overall fitness of an individual is controlled and restricted by finite levels of energy allocation
Decreased gametogenesis occurred in the northern cricket frog after antigenic stimulation [4], in mice exposed to Toxoplasma gondii [5], in cattle with viral diarrhoea [6] and pneumonia [7], in dogs with canine leishmaniasis [8], and in humans with Helicobacter pylori infection [9] and AIDS [10,11]
In other cases, there is no loss in reproductive effort; for example, gametogenesis is unaffected by asymptomatic HIV infection in humans [15]
Summary
Overall fitness of an individual is controlled and restricted by finite levels of energy allocation. Life-history trade-offs in energy allocation between physiological processes such as reproduction and fighting infectious disease are fundamental to fitness [1]. Mounting an immune response is costly, and to effectively fight infection, a trade-off of resources occurs [2]. Many studies have characterized infections that lead to a reduction in various reproductive measurements, like gametogenesis (gamete development), fertility and parental care [3]. Decreased overall fertility has been reported in the insect Rhodnius prolixus when infected with Trypanosoma rangeli [12], and decreased parental care effort has been observed in blue tits [13], and in house sparrows as they combat infection [14]. In other cases, there is no loss in reproductive effort; for example, gametogenesis is unaffected by asymptomatic HIV infection in humans [15]. Subclinical infections of Cowdria ruminantium in ewes did not affect female fertility [16], and rate of conception was unaffected in humans with Behcet’s syndrome (a rare autoimmune disorder that causes inflammation of the blood vessels) [17]
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