Testosterone and 11-ketotestosterone have different regulatory effects on electric communication signals of male Brachyhypopomus gauderio

Evolution of androgen receptors contributes to species variation in androgenic regulation of communication signals in electric fishes

Hormones and receptors coevolve to generate species diversity in hormone action. We compared the structure and function of androgen receptors (ARs) across fishes, with a focus on ARs in ghost knifefishes (Apteronotidae). Apteronotids, like many other teleosts, have two ARs (ARα and ARβ). ARβ is largely conserved, whereas ARα sequences vary considerably across species. The ARα ligand binding domain (LBD) has evolved under positive selection, and differences in the LBD across apteronotid species are associated with diversity in androgenic regulation of behavior. The Apteronotus leptorhynchus ARα LBD differs substantially from that of the Apteronotus albifrons ARα or the ancestral AR. Structural modeling and transactivation assays demonstrated that A. leptorhynchus ARα cannot bind androgens. We propose a model whereby relative expression of ARα versus ARβ in the brain, coupled with loss of androgen binding by ARα in A. leptorhynchus might explain reversals in androgenic regulation and sex differences in electrocommunication behavior.

Bioactivity of 11 keto and hydroxy androgens in yeast and mammalian host cells

Recent studies have highlighted the potential role of 11oxygenated (keto or hydroxy) androgens in human reproductive function with 11keto androgens circulating at concentrations comparable with testosterone in women and children. However, the intrinsic androgenic bioactivities of 11 keto and hydroxy androgens are not fully characterized. We therefore investigated the full androgen dose-response curves using complementary in vitro yeast and mammalian (HEK293) host cell bioassays of 11 keto and hydroxy derivatives of the potent androgens, testosterone (T) and dihydrotestosterone (DHT), compared with their parent non-11 oxygenated steroids together with the pro-androgen precursor (androstenedione (A₄)) and metabolites (androstanedione, androsterone). For potent androgens, the mammalian HEK293 host cell bioassay was 22-138 times more sensitive than the yeast host cell bioassay. In both androgen bioassays, 11keto derivatives displayed androgenic bioactivity but significantly lower molar potency than their parent non-keto steroids. By contrast, the 11hydroxy derivatives had minimal or no androgenic bioactivity. In both bioassays 5α-reduction increased androgenic potency. These findings confirm that that 11keto androgens may contribute directly to androgen status in women, children, and other conditions apart from healthy eugonadal men whereas 11hydroxy androgens have negligible androgenic potency although it cannot be excluded that they may be converted to more potent androgens in vivo.

Electroreception, electrogenesis and electric signal evolution

Electroreception, the capacity to detect external underwater electric fields with specialised receptors, is a phylogenetically widespread sensory modality in fishes and amphibians. In passive electroreception, a capacity possessed by c. 16% of fish species, an animal uses low-frequency-tuned ampullary electroreceptors to detect microvolt-range bioelectric fields from prey, without the need to generate its own electric field. In active electroreception (electrolocation), which occurs only in the teleost lineages Mormyroidea and Gymnotiformes, an animal senses its surroundings by generating a weak (< 1 V) electric-organ discharge (EOD) and detecting distortions in the EOD-associated field using high-frequency-tuned tuberous electroreceptors. Tuberous electroreceptors also detect the EODs of neighbouring fishes, facilitating electrocommunication. Several other groups of elasmobranchs and teleosts generate weak (< 10 V) or strong (> 50 V) EODs that facilitate communication or predation, but not electrolocation. Approximately 1.5% of fish species possess electric organs. This review has two aims. First, to synthesise our knowledge of the functional biology and phylogenetic distribution of electroreception and electrogenesis in fishes, with a focus on freshwater taxa and with emphasis on the proximate (morphological, physiological and genetic) bases of EOD and electroreceptor diversity. Second, to describe the diversity, biogeography, ecology and electric signal diversity of the mormyroids and gymnotiforms and to explore the ultimate (evolutionary) bases of signal and receptor diversity in their convergent electrogenic-electrosensory systems. Four sets of potential drivers or moderators of signal diversity are discussed. First, selective forces of an abiotic (environmental) nature for optimal electrolocation and communication performance of the EOD. Second, selective forces of a biotic nature targeting the communication function of the EOD, including sexual selection, reproductive interference from syntopic heterospecifics and selection from eavesdropping predators. Third, non-adaptive drift and, finally, phylogenetic inertia, which may arise from stabilising selection for optimal signal-receptor matching.

Electrocyte physiology: 50 years later

Weakly electric gymnotiform and mormyrid fish generate and detect weak electric fields to image their worlds and communicate. These multi-purpose electric signals are generated by electrocytes, the specialized electric organ (EO) cells that produce the electric organ discharge (EOD). Just over 50 years ago the first experimental analyses of electrocyte physiology demonstrated that the EOD is produced and shaped by the timing and waveform of electrocyte action potentials (APs). Electrocytes of some species generate a single AP from a distinct region of excitable membrane, and this AP waveform determines EOD waveform. In other species, electrocytes possess two independent regions of excitable membrane that generate asynchronous APs with different waveforms, thereby increasing EOD complexity. Signal complexity is further enhanced in some gymnotiforms by the spatio-temporal activation of distinct EO regions with different electrocyte properties. For many mormyrids, additional EOD waveform components are produced by APs that propagate along stalks that connect postsynaptic regions to the main body of the electrocyte. I review here the history of research on electrocyte physiology in weakly electric fish, as well as recent discoveries of key phenomena not anticipated during early work in this field. Recent areas of investigation include the regulation of electrocyte activity by steroid and peptide hormones, the molecular evolution of electrocyte ion channels, and the evolutionary selection of ion channels expressed in excitable cells. These emerging research areas have generated renewed interest in electrocyte function and clear future directions for research addressing a broad range of new and important questions.

Behavioral ecology, endocrinology and signal reliability of electric communication

The balance between the costs and benefits of conspicuous animal communication signals ensures that signal expression relates to the quality of the bearer. Signal plasticity enables males to enhance conspicuous signals to impress mates and competitors and to reduce signal expression to lower energetic and predation-related signaling costs when competition is low. While signal plasticity may benefit the signaler, it can compromise the reliability of the information conveyed by the signals. In this paper we review the effect of signal plasticity on the reliability of the electrocommunication signal of the gymnotiform fish Brachyhypopomus gauderio. We (1) summarize the endocrine regulation of signal plasticity, (2) explore the regulation of signal plasticity in females, (3) examine the information conveyed by the signal, (4) show how that information changes when the signal changes, and (5) consider the energetic strategies used to sustain expensive signaling. The electric organ discharge (EOD) of B. gauderio changes in response to social environment on two time scales. Two hormone classes, melanocortins and androgens, underlie the short-term and long-term modulation of signal amplitude and duration observed during social interaction. Population density drives signal amplitude enhancement, unexpectedly improving the reliability with which the signal predicts the signaler's size. The signal's second phase elongation predicts androgen levels and male reproductive condition. Males sustain signal enhancement with dietary intake, but when food is limited, they 'go for broke' and put extra energy into electric signals. Cortisol diminishes EOD parameters, but energy-limited males offset cortisol effects by boosting androgen levels. While physiological constraints are sufficient to maintain signal amplitude reliability, phenotypic integration and signaling costs maintain reliability of signal duration, consistent with theory of honest signaling.

Cellular mechanisms of developmental and sex differences in the rapid hormonal modulation of a social communication signal

Some gymnotiform electric fish species rapidly modify their electric signal waveforms by altering the action potential (AP) waveforms of their electrocytes, the excitable cells that produce the electric organ discharge (EOD). The fish Brachyhypopomus gauderio alters EOD amplitude and pulse duration as a social signal in accordance with the prevailing social conditions, under the dual regulation of melanocortins and androgens. We show here that B. gauderio uses two distinct cellular mechanisms to change signal amplitude, and its use of these two mechanisms varies with age and sex of the signaler. EOD amplitude and waveform are regulated by the coordinated timing and shaping of two APs generated from two opposing excitable membranes in each electrocyte. The two membranes fire in sequence within 100 μs of each other with the second AP being broader than the first. We have shown previously that mature males increase EOD amplitude and duration when melanocortin peptide hormones act directly on electrocytes to selectively broaden the second AP and increase the delay between the two APs by approximately 25 μs. Here we show that females selectively broaden only the second AP as males do, but increase amplitude of both APs with no change in delay between them, a previously unreported second mechanism of EOD amplitude change in B. gauderio. Juvenile fish broaden both APs and increase the delay between the APs. Cellular mechanisms of EOD plasticity are therefore shaped during development, presumably by sex steroids, becoming sexually dimorphic at maturity.

Food restriction promotes signaling effort in response to social challenge in a short-lived electric fish

Vertebrates exposed to stressful conditions release glucocorticoids to sustain energy expenditure. In most species elevated glucocorticoids inhibit reproduction. However individuals with limited remaining reproductive opportunities cannot afford to forgo reproduction and should resist glucocorticoid-mediated inhibition of reproductive behavior. The electric fish Brachyhypopomus gauderio has a single breeding season in its lifetime, thus we expect males to resist glucocorticoid-mediated inhibition of their sexual advertisement signals. We studied stress resistance in male B. gauderio (i) by examining the effect of exogenous cortisol administration on the signal waveform and (ii) by investigating the effect of food limitation on androgen and cortisol levels, the amplitude of the electric signal waveform, the responsiveness of the electric signal waveform to social challenge, and the amount of feeding activity. Exogenous cortisol administration did reduce signal amplitude and pulse duration, but endogenous cortisol levels did not rise with food limitation or social challenge. Despite food limitation, males responded to social challenges by further increasing androgen levels and enhancing the amplitude and duration of their electric signal waveforms. Food-restricted males increased androgen levels and signal pulse duration more than males fed ad libitum. Socially challenged fish increased food consumption, probably to compensate for their elevated energy expenditure. Previous studies showed that socially challenged males of this species simultaneously elevate testosterone and cortisol in proportion to signal amplitude. Thus, B. gauderio appears to protect its cortisol-sensitive electric advertisement signal by increasing food intake, limiting cortisol release, and offsetting signal reduction from cortisol with signal-enhancing androgens.

Signal modulation as a mechanism for handicap disposal

Signal honesty may be compromised when heightened competition provides incentive for signal exaggeration. Some degree of honesty might be maintained by intrinsic handicap costs on signalling or through imposition of extrinsic costs, such as social punishment of low quality cheaters. Thus, theory predicts a delicate balance between signal enhancement and signal reliability that varies with degree of social competition, handicap cost, and social cost. We investigated whether male sexual signals of the electric fish Brachyhypopomus gauderio would become less reliable predictors of body length when competition provides incentives for males to boost electric signal amplitude. As expected, social competition under natural field conditions and in controlled lab experiments drove males to enhance their signals. However, signal enhancement improved the reliability of the information conveyed by the signal, as revealed in the tightening of the relationship between signal amplitude and body length. Signal augmentation in male B. gauderio was independent of body length, and thus appeared not to be curtailed through punishment of low quality (small) individuals. Rather, all individuals boosted their signals under high competition, but those whose signals were farthest from the predicted value under low competition boosted signal amplitude the most. By elimination, intrinsic handicap cost of signal production, rather than extrinsic social cost, appears to be the basis for the unexpected reinforcement of electric signal honesty under social competition. Signal modulation may provide its greatest advantage to the signaller as a mechanism for handicap disposal under low competition rather than as a mechanism for exaggeration of quality under high competition.

Tight hormonal phenotypic integration ensures honesty of the electric signal of male and female Brachyhypopomus gauderio

Hormones mediate sexually selected traits including advertisement signals. Hormonal co-regulation links the signal to other hormonally-mediated traits such that the tighter the integration, the more reliable the signal is as a predictor of those other traits. Androgen administration increases the duration of the communication signal pulse in both sexes of the electric fish Brachyhypopomus gauderio. To determine whether the duration of the signal pulse could function as an honest indicator of androgen levels and other androgen-mediated traits, we measured the variation in sex steroids, signal pulse duration, and sexual development throughout the breeding season of B. gauderio in marshes in Uruguay. Although the sexes had different hormone titres and signal characteristics, in both sexes circulating levels of the androgens testosterone (T) and 11-ketotestosterone (11-KT) were strongly related to signal pulse duration. Consequently, signal pulse duration can serve as an honest indicator of circulating androgens in males and females alike. Additionally, through phenotypic integration, signal pulse duration also predicts other sexual traits directly related to androgen production: gonad size in males and estradiol (E2) levels in females. Our findings show that tight hormonal phenotypic integration between advertisement signal and other sex steroid-mediated traits renders the advertisement signal an honest indicator of a suite of reproductive traits.