Neuroendocrine mechanisms underlying sexual plasticity in teleosts - Research IBBG

In species in which intense inter-male competition for the access to females is present, males of lower competitive ability may adopt alternative reproductive tactics (ART) to get access to mates. These ART translate in many cases into male sexual polymorphism, with individuals following different tactics looking very distinct. Usually two alternative male morphs can be recognized in species with ART: 1) bourgeois males that compete for access to mates, invest in male displaying characters, such as building elaborated nests or expressing ornaments; and 2) parasitic males that take advantage of the success of the bourgeois males in attracting females and attempt sneaking fertilizations (e.g. sneaker and satellite males). Because males are usually the competing sex sexual polymorphism occurs mainly among males. Within the vertebrates, teleost fishes are the taxa with the larger number of sexually polymorphic species that has been described. The alternative tactics may be fixed or flexible. In species with flexible tactics the individuals switch between tactics in a frequency or condition dependent fashion (e.g. relative size or age). In fixed tactics alternative phenotypes remain the same for the whole life span, and may be genetic or environment (e.g. social status or density) determined. The occurrence of ART and their diversity offers the possibility for an insight into the endocrine mechanisms underlying reproductive behaviours, since in the parasitic tactic the expression of behavioural and morphological male traits is dissociated from gonad maturation and sperm production. Thus alternative morphs can almost be viewed as sexual mosaics that may express simultaneously male and female phenotypic traits. Moreover, in species in which the expression of the tactic is open to the influence of the social environment they allow the study of how social stimuli are tranduced into signals that modulate the activity of the reproductive axis in its multiple facets.

 

Two hormone families are good candidates for the investigation of the neuroendocrine mechanisms of ART:

 

(a) Sex steroids (i.e. androgens, estrogens and progestins), since they are produced by the gonads they can be viewed as honest signals of gonad maturation (i.e. readiness for breeding) for the organism. In other words it would make no sense for an individual to invest in displaying reproductive behaviour and to express secondary sex characters if its gonads were not ready for gamete production. Currently it is known that in teleost species with ART bourgeois males have higher plasma levels of 11-keto-testosterone (KT) but not of testosterone (T). The relationship between estrogens and progestins and the expression of ART in teleosts remains to be investigated. It has been hypothesised that depending on the tactic being fixed or flexible the steroid effects could be different, with activational effects predicted in species with flexible tactics and organisational effects predicted in species with fixed tactics.

 

(b) Neuropeptides (e.g. gonadotrophin releasing hormone - GnRH, arginine vasotocin – AVT, isotocin), since they are involved in central pathways that control the activity of the reproductive axis (i.e. GnRH) and the expression of reproductive behaviours in non-mammalian vertebrates (i.e. AVT). In species with ART it has been shown that GnRH-ir neurons in the POA are present in both male morphs and that there are no significant differences in their number or soma size. In species with male ART, bourgeois males can be predicted to have higher AVT levels than parasitic males, since the bourgeois males are usually the courting morph. In the 3 species with ART for which AVT data is available so far, bourgeois males have either larger soma of AVT-ir neurons or larger numbers of AVT mRNA producing cells in the preoptic area (POA) than parasitic males (Porichthys notatus, Thalassoma dupery, and T. bifasciatum).

 

We have two ongoing research projects addressing questions within this research line:

 

- “Hormones and life-history trade-offs and plasticity: a study on alternative reproductive tactics in blennid fish” POCTI 38395/BSE/2001.

 

- “Reproductive biology of the blenny Salaria pavo from Ria Formosa: knowledge for population management and conservation strategies” PNAT/BIA/15090/99.

 

We have also already finished a first project on this topic: “Neuroethology and behavioural endocrinology of male sexual polymorphism in the Azorean rock-pool blenny Parablennius sanguinolentus parvicornis (Pisces: Blenniidae)” PRAXIS PCNA/C/BIA/94/96.

 

So far we have documented androgen levels for a set of species with fixed and with flexible ART. We have shown that androgen levels are higher in bourgeois males than in parasitic males of the peacock blenny, the rock-pool blenny (both with flexible ART) and the sailfin molly (with fixed ART). We have also administered exogenous androgens to parasitic males of these different species to see if we could induce the switch between tactics expected in the case of species with flexible tactics but not in the case of species with fixed ART. According to the predictions we were able to inhibit the expression of female traits and to promote some male traits in parasitic males of the two blenniid species (both with flexible ART) but not in the case of the sailfin molly (with fixed ART, in which case an organisational but not an activational effect is to be expected).

 

We have also started to investigate the role of AVT in the expression of courtship behaviour vs. the expression of the alternative morphotype. As mentioned above in all the species with ART studied so far bourgeois males had larger or more numerous POA AVT-ir cells. However, these differences can be attributed either to the differences in the expression of alternative morphotypes or to the differences in the expression of courtship behavior between morphotypes. Therefore, neuroendocrine correlates of ART can only be established in species in which the expression of the bourgeois tactic is decoupled from the expression of courtship behavior. This is the case of a population of the peacock blenny living in a coastal lagoon in Southern Portugal in which the scarcity of nest sites has lead to sex-role reversal in courtship behavior. Our group has recently published the results of cellular and molecular studies on the neuroendocrine correlates of male sexual polymorphism in this population. Immunocytochemistry and in situ hybridization were used to examine the production of arginine vasotocin (AVT) peptide and messenger RNA, respectively. The expression of AVT mRNA on a per cell basis was correlated with mating behavior, rather than sex morphotype, suggesting that the greater level of AVT mRNA expression in females and sneakers is correlated with the production of courtship behavior. On the other hand, the number and size of AVT peptide producing cells in both male types is higher or larger, respectively, than in females suggesting that it is correlated with sex morphotype, rather than mating behavior. Moreover systemic injections (i.e. inter-peritoneal) of AVT promoted the expression of female-like courtship behaviour in sneakers and females but failed to induce male-courtship behaviour in bourgeois males.

 

In collaboration with the lab of Adelino Canário and Deborah Power (University of Algarve), we started to develop molecular approaches have started to allow the study of these systems from two new perspectives:

 

(a) most people have been looking at the endocrine basis of ART just by assaying sex steroid levels. We feel it is very important also to look at these systems from the perspective of the receptors. In fact similar circulating levels may have significant differences in their effects depending on localised differences in receptor densities. So far only one study has addressed this problem using quantitative RT-PCR and it was found that bourgeois males had higher density of androgen receptors (AR) in the brain than sneakers but that there was no differences in the gonads. Thus we are currently studying AR and estrogen receptors (ER, since androgen effects may be mediated by estrogens after androgen aromatisation) in the different male morphs using RT-PCR and/or in situ hybridisation (depending on the specific goals of the study).

 

(b) to look at other hormones than sex steroids, namely to gonadotrophins and to prolactin-family neuropeptides that might mediate the trade-off between growth and reproduction that alternative morphs experience differentially. In a cichlid fish in which ART does not occur but in which the social status of the individual affects the activity of its reproductive axis it has been shown at the molecular level a trade-off between growth and reproduction with dominant males expressing GnRH and subordinates somatostatin. We propose to characterise these two neuropeptide families in alternative morphs in species with fixed vs flexible strategies using RT-PCR and/or in situ hybridization techniques. We also plan to manipulate the social environment and to track changes in these systems that may mediate the switch between ART in species with flexible ART.

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