Earlier sex change in infected individuals of the protogynous reef fish Thalassoma bifasciatum

Hermaphrodites and parasitism: size-specific female reproduction drives infection by an ephemeral parasitic castrator

Sex can influence patterns of parasitism because males and females can differ in encounter with, and susceptibility to, parasites. We investigate an isopod parasite (Hemioniscus balani) that consumes ovarian fluid, blocking female function of its barnacle host, a simultaneous hermaphrodite. As a hermaphrodite, sex is fluid, and individuals may allocate energy differentially to male versus female reproduction. We predicted the relationship between barnacle size and female reproductive function influences the distribution of parasites within barnacle populations. We surveyed 12 populations spanning ~400 km of coastline of southern California and found intermediate-sized barnacles where most likely to be actively functioning as females. While it is unclear why larger individuals are less likely to be actively reproducing as females, we suggest this reduced likelihood is driven by increased investment in male reproductive effort at larger sizes. The female function-size relationship was mirrored by the relationship between size and parasitism. We suggest parasitism by Hemioniscus balani imposes a cost to female function, reinforcing the lack of investment in female function by the largest individuals. Within the subset of suitable (=female) hosts, infection probability increased with size. Hence, the distribution of female function, combined with selection for larger hosts, primarily dictated patterns of infection.

Structural and molecular characterization of Kudoa quraishii n. sp. from the trunk muscle of the Indian mackerel Rastrelliger kanagurta (Perciforme, Scombridae) in Saudi Arabia coasts

A new Myxozoa, Kudoa quraishii n. sp., is reported in the striated muscle of the Indian mackerel Rastrelliger kanagurta from the Red Sea and the Arabian Gulf in Saudi Arabia. Mean prevalence of infection is about 20% and varies between localities. The parasite develops whitish and oval or rounded pseudocysts of 0.2-3 mm in the striated muscles of the body. Pseudocysts are filled with mature spores. Myxospores are quadrate in shape in apical view with rounded edges and ovoid in side view. Each spore is formed by four equal shell valves and four symmetrical polar capsules. Polar capsules are pyriform in apical view and drop-like in side view. Myxospore measurements in micrometers are 6.14 (5.9-6.34) in width, 5.48 (5.3-5.71) in thickness, and 4.27 (4.1-4.42) in length. Polar capsule measurements in apical view in micrometers are 2.08 (1.88-2.28) and 1.31 (1.10-1.52) length by width. Molecular analysis based on SSU rDNA gene shows closest association with K. amamiensis and K. kenti with respectively 98 and 97.2% of similarities.

Kudoa azevedoi n. sp. (Myxozoa, Multivalvulida) from the oocytes of the Atlantic horse mackerel Trachurus trachurus (Perciformes, Carangidae) in Tunisian coasts

A new species Kudoa azevedoi sp. n. (Myxozoa, Multivalvulida) is described in Trachurus trachurus Linnaeus, 1758 (Carangidae) from fishing harbors in Tunisian coasts using spore morphology and SSU rDNA sequence data. The parasite occurs only in ovaries within oocytes of mature and immature specimens. Spores are quadrate in shape in apical view with rounded edges, having four shell valves and four symmetrical polar capsules. They are of small sizes and measure 3.5±0.41 (3-4.2)×4.5±0.44 (4-5.2) length by width. The polar capsules are pyriform in shape measuring 1.5±0.22 (1.5-2)×0.75±0.14 (0.5-1) μm. Infected oocytes are hypertrophied, whitish colored, and filled with mature spores. Plasmodia are tubular and ramified from the inner membrane toward the center of the oocyte. Phylogenetic analysis based on small subunit ribosomal DNA sequences shows the highest similarity (96%) with the ovary parasite Kudoa ovivora. Some morphological details and spore dimensions support the creation of a new species in the genus Kudoa. Mean prevalence among examined females is of about 55.5%. It varies between localities and length of fish. The present myxosporea is the second Kudoa species reported in fish ovaries.

Host sexual dimorphism and parasite adaptation

Disease expression and prevalence often vary in the different sexes of the host. This is typically attributed to innate differences of the two sexes but specific adaptations by the parasite to one or other host sex may also contribute to these observations.

In species with separate sexes, parasite prevalence and disease expression is often different between males and females. This effect has mainly been attributed to sex differences in host traits, such as immune response. Here, we make the case for how properties of the parasites themselves can also matter. Specifically, we suggest that differences between host sexes in many different traits, such as morphology and hormone levels, can impose selection on parasites. This selection can eventually lead to parasite adaptations specific to the host sex more commonly encountered, or to differential expression of parasite traits depending on which host sex they find themselves in. Parasites adapted to the sex of the host in this way can contribute to differences between males and females in disease prevalence and expression. Considering those possibilities can help shed light on host–parasite interactions, and impact epidemiological and medical science.

Classic predictions about sex change do not hold under all types of size advantage

Theory predicts that the 'size advantage' (rate of increase in male and female fitness with age or size) determines the direction and the timing of sex change in sequential hermaphrodites. Whereas the size advantage is generated by the mating system and would be expected to vary within and between species, the shape or form of the size advantage has rarely been estimated directly. Here, we ask whether theoretical predictions about the timing of sex change hold under different types of size advantage. We model two biological scenarios representing different processes generating the size advantage and find that different types of size advantage can produce patterns that qualitatively differ from classic predictions. Our results demonstrate that a good understanding of sequentially hermaphroditic mating systems, and specifically, a direct assessment of the processes underlying the size advantage is crucial to reliably predict and explain within-species patterns of the timing of sex change.

Can myxosporean parasites compromise fish and amphibian reproduction?

Research into fish and amphibian reproduction has increased exponentially in recent years owing to the expansion of the aquaculture industry, the need to recover fishery populations, the impact of endocrine disruptors on the aquatic environment and the global decline of amphibian populations. This review focuses on a group of parasites, the Myxozoa, that affect fish and amphibian reproduction. Lists of the myxosporeans that specifically infect gonads are provided. Most of these are parasitic of freshwater hosts, and most amphibian cases are reported from testes. Sex specificity and sex reversal are discussed in relation to gonadal parasitism. The immune response of the fish to the infection is described, and the contribution of the immunoprivilege of gonads to host invasion is emphasized. The pathological effect of these parasites can be significant, especially in aquacultured broodstocks, on some occasions, leading to parasitic castration. Although myxosporean parasites are currently not very frequent in gonads, their impact could increase in the future owing to the transactions in the global market. Their easy release into the aquatic environment with spawning could make their spreading even more feasible. In the absence of commercial drugs or vaccines to treat and prevent these infections, there is an urgent need to develop specific, rapid and reliable diagnostic tools to control and manage animal movements. In addition, much effort is still to be made on deciphering the life cycle of these organisms, their invasion strategies and their immune evasion mechanisms.

Tests of sex allocation theory in simultaneously hermaphroditic animals

Sex allocation is a crucial life-history parameter in all sexual organisms. Over the last decades a body of evolutionary theory, sex allocation theory, was developed, which has yielded capital insight into the evolution of optimal sex allocation patterns and adaptive evolution in general. Most empirical work, however, has focused on species with separate sexes. Here I review sex allocation theory for simultaneous hermaphrodites and summarize over 50 empirical studies, which have aimed at evaluating this theory in a diversity of simultaneous hermaphrodites spanning nine animal phyla. These studies have yielded considerable qualitative support for several predictions of sex allocation theory, such as a female-biased sex allocation when the number of mates is limited, and a shift toward a more male-biased sex allocation with increasing numbers of mates. In contrast, many fundamental assumptions, such as the trade-off between male and female allocation, and numerous predictions, such as brooding limiting the returns from female allocation, are still poorly supported. Measuring sex allocation in simultaneously hermaphroditic animals remains experimentally demanding, which renders evaluation of more quantitative predictions a challenging task. I identify the main questions that need to be addressed and point to promising avenues for future research.

Male-biased sex-ratio distortion caused by Octosporea bayeri, a vertically and horizontally-transmitted parasite of Daphnia magna

Female-biased sex-ratio distortion is often observed in hosts infected with vertically-transmitted microsporidian parasites. This bias is assumed to benefit the spread of the parasite, because male offspring usually do not transmit the parasite further. The present study reports on sex-ratio distortion in a host-parasite system with both horizontal and vertical parasite transmission: the microsporidium Octosporea bayeri and its host, the planktonic cladoceran Daphnia magna. In laboratory and field experiments, we found an overall higher proportion of male offspring in infected than in uninfected hosts. In young males, there was no parasite effect on sperm production, but, later in life, infected males produced significantly less sperm than uninfected controls. This shows that infected males are fertile. As males are unlikely to transmit the parasite vertically, an increase in male production could be advantageous to the host during phases of sexual reproduction, because infected mothers may obtain uninfected grandchildren through their sons. Life-table experiments showed that, overall, sons harboured more parasite spores than their sisters, although they reached a smaller body size and died earlier. Male production may thus be beneficial for the parasite when horizontal transmission has a large pay-off as males may contribute more effectively to parasite spread than females.

Resource-dependent sex-allocation in a simultaneous hermaphrodite

Most sex allocation theory is based on the relationship between the resource investment into male and female reproduction and the consequent fitness returns (often called fitness-gain curves). Here we investigate the effects of resource availability on the sex allocation of a simultaneously hermaphroditic animal, the free-living flatworm Macrostomum lignano. We kept the worms under different resource levels and determined the size of their testes and ovaries over a period of time. At higher resource levels, worms allocated relatively more into the female function, suggesting a saturating male fitness-gain curve for this species. A large part of the observed effect was due to a correlated increase in body size, showing size-dependent sex allocation in M. lignano. However, a significant part of the overall effect was independent of body size, and therefore likely due to the differences in resource availability. Moreover, in accordance with a saturating male fitness-gain curve, the worms developed the male gonads first. As the group size was kept constant, our results contrast with expectations from sex allocation models that deal with local mate competition alone, and with previous experiments that test these models.

A Dimensionless Invariant for Relative Size at Sex Change in Animals: Explanation and Implications

Recent comparative studies across sex-changing animals have found that the relative size and age at sex change are strikingly invariant. In particular, 91%-97% of the variation in size at sex change across species can be explained by the simple rule that individuals change sex when they reach 72% of their maximum body size. However, this degree of invariance is surprising and has proved controversial. In particular, it is not clear why this result should hold, given that there is considerable biological variation across species in factors that can influence the evolutionarily stable timing of sex change. Our overall aim here is to explain this result and determine the implications for other life-history variables. Specifically, we use a combination of approaches to formalize and make explicit previous analytical theory in this area, examine the robustness of the empirical invariance result, and carry out sensitivity analyses to determine what the empirical data imply about the mean value and variation in several key life-history variables.