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Sexual selection in Poecilia reticulata: the maintenance of variability in male pre- and postcopulatory sexual traits
Full text linkSexual selection is a driving force in sexually reproducing organisms and strongly shapes their evolution. In the last three decades, sexual selection research has seen a rapid growth, and both theoretical and empirical work has clarified many components of pre- and postcopulatory sexual selection. Despite that, the coexistence of two basic observations still forms an unsolved evolutionary question: in natural populations genetic variation is found in almost all traits in the presence of strong natural and sexual selection. As selection should deplete variability those two observations are in direct conflict. This problem attracted the attention of many researchers, as it regards potentially most of the numerous traits describing an organism’s phenotype, or at least all the traits under some selection. During my PhD I explored part of this field of study, focusing on sexually selected male traits. Most of the efforts done to understand this evolutionary contradiction have been done in a precopulatory context, with particular attention to the prominent case of the so called ‘lek-paradox’. However, whenever females are sexually promiscuous, a directional selection for traits associated with sperm competition success is expected to arise. As ejaculate characteristics are expected, and actually known, to play a crucial role in determining the fitness outcome of males, selection acting on them should be strong and as a consequence their variability reduced. Yet, as for precopulatory traits, there are many experimental evidences that variability in postcopulatory traits is unexpectedly high.
Many hypotheses have been formulated to explain the maintenance of genetic variability of sexually selected traits. During my PhD I tested some prediction of three main models applicable to both pre- and postcopulatory traits: first that selection constrains and non linear selection are acting on the set of traits defining the male phenotype. Second, I verified that resource trade-offs are present between pre- and postcopulatory traits, as proposed by Parker’s sperm competition theory. Third, I tested the fundamental assumption of the ‘genic capture hypothesis’ that sexually selected traits are condition dependent.
I performed four main experiments using the guppy, Poecilia reticulata. This small tropical fresh-water fish is well suited for my purposes as traits subject to both pre- (male ornamentation, size, and behaviour) and postcopulatory selection (sperm number, velocity, and viability) exhibit high levels of phenotypic and additive genetic variation. With the first experiment (manuscript 1) I characterized, for the first time, the selection acting in a whole on both pre- and postcopulatory traits. I then measured the long term cost of sperm production (manuscript 2) with the aim of determine the trade-offs present between pre- and postcopulatory traits. With the last two experiments (manuscripts 3 and 4) I tested condition dependence of a wide set of sexually selected traits.
My results suggest that in this species non linear selection may be more important than previously estimated and, in particular, that disruptive and correlational selection can contribute to maintain polymorphisms in sexually selected traits. Moreover investment in ejaculate is traded off with investment in obtaining mating, in agreement with sperm competition theory. Lastly, both pre- and postcopulatory sexually selected traits show a strong condition dependence, thus confirming one assumption of the ‘genic capture hypothesis’.La selezione sessuale, descritta da Darwin nella sua opera “The Descent of Man and Selection in Relation to Sex”(1871) è una delle forze trainanti in natura e, nella maggior parte degli esseri viventi, determina fortemente la loro evoluzione. Negli ultimi tre decenni, la ricerca scientifica nell’ambito della selezione sessuale ha visto una rapida crescita, e numerosissimi lavori sia teorici che sperimentali hanno chiarito numerosi aspetti della selezione sessuale sia pre- che postcopulatoria. Nonostante ciò, la coesistenza di due fondamentali condizioni in natura fa si che esista ancora un paradosso evolutivo irrisolto: nella maggior parte dei caratteri si osserva una grande variabilità genetica nonostante la presenza di una forte selezione, sia naturale che sessuale. Poiché la selezione dovrebbe esaurire la variabilità genetica, queste due condizioni (variabilità da una parte e selezione dall’altra) sono in diretto conflitto. Poiché questa situazione riguarda la maggior parte dei tratti che formano il fenotipo di un organismo, o almeno tutti quei caratteri sotto una qualche forma di selezione, è facilmente immaginabile come questo problema abbia attirato l’attenzione di moltissimi ricercatori.
Durante il mio dottorato di ricerca ho mi sono dedicato allo studio di una parte di questo problema, concentrandomi sui caratteri maschili selezionati sessualmente. Questo campo è stato ampliamente studiato ma la maggior parte degli sforzi fatti per comprendere questa contraddizione è stata compiuta esclusivamente in un contesto di selezione precopulatoria, ed in particolare per quanto riguarda un suo caso particolare, quello del paradosso del lek.
Tuttavia, nel caso in cui le femmine di una specie siano sessualmente promiscue (situazione quasi totalmente diffusa nel regno animale), ci si aspetta la presenza di una selezione direzionale per i caratteri maschili legati alla competizione spermatica. Ed infatti quello che si osserva è che le caratteristiche dell’eiaculato hanno un ruolo importante nel determinare il successo riproduttivo maschile. La selezione su questi tratti è quindi forte ma, come per i caratteri precopulatori, numerose evidenze sperimentali dimostrano la presenza di un’elevata variabilità sia genetica che fenotipica in caratteri soggetti a selezione postcopulatoria.
Molte ipotesi sono state formulate per spiegare il mantenimento della variabilità genetica nei tratti selezionati sessualmente. In particolare, durante il mio dottorato ho testato le previsioni di tre di queste principali teorie, applicabili sia ai tratti pre- che postcopulatori. Per prima cosa ho verificato la presenza di selezione non lineare disruptiva e correlazionale prendendo in considerazione un ampio set di caratteri maschili. In secondo luogo, ho verificato che esistano dei trade-off di tipo energetico tra i tratti pre- e postcopulatori, come proposto nella ‘teoria della competizione spermatica’ di Parker. Infine, ho testato una delle condizioni fondamentali della teoria della ‘cattura genica’ proposta da Rowe e Houle (1996) e cioè che i tratti selezionati sessualmente siano condizione-dipendenti.
Durante il dottorato ho svolto quattro esperimenti principali utilizzando come specie modello Poecilia reticulata, comunemente chiamata guppy. Questo piccolo pesce tropicale d'acqua dolce è particolarmente adatto per i miei scopi. I maschi presentano infatti caratteristiche soggette sia a selezione precopulatoria (ornamenti, dimensioni e comportamento sessuale) che a selezione postcopulatoria (numero, velocità e vitalità degli spermatozoi) ed inoltre si osserva in questi caratteri un’elevata varianza genetica additiva. Appare quindi evidente la presenza della contraddizione prima descritta. Con il primo esperimento (primo articolo) ho descritto, per la prima volta, la selezione non lineare che agisce sull’insieme dei tratti sia pre- che postcopulatori. Ho poi misurato il costo a lungo termine imposto dalla produzione di spermi (secondo articolo) con l'obiettivo di determinare i trade-off presenti tra i tratti pre- e postcopulatori. Negli ultimi due articoli (terzo e quarto) ho testato l’ipotesi di condizione-dipendenza in un ampio set di tratti selezionati sessualmente.
I miei risultati suggeriscono che in questa specie la selezione non lineare può essere più importante di quanto stimato in precedenza e, in particolare, che la selezione disruptiva e correlazionale possono contribuire a mantenere il polimorfismo osservato nei tratti selezionati sessualmente. Inoltre l’investimento a livello postcopulatorio nell’eiaculato presenta per i maschi di Poecilia un costo in termini di successo precopulatorio (possibilità di accoppiarsi), in accordo con la teoria della competizione spermatica. Infine, i tratti sessualmente selezionati, sia pre- che postcopulatori, mostrano una forte dipendenza dalla condizione del maschio, confermando così uno degli assunti dell’ ipotesi della cattura genica
Sexual selection and ageing: Interplay between pre- and post-copulatory traits senescence in the guppy
No full textTraits associated with mating and fertilization success are expected to senesce with age, but limited information is available on their relative rates of senescence. In polyandrous species, male reproductive fitness depends on both mating and fertilization success. Because successful mating is a prerequisite for post-copulatory sexual selection, ejaculate traits are expected to senesce faster than pre-copulatory traits, as precopulatory sexual selection is often deemed to be stronger than post-copulatory sexual selection. This pattern has generally been found in the few empirical studies conducted so far. We tested this prediction in the guppy (Poecilia reticulata), a livebearing fish characterized by intense sperm competition, by comparing the expression of male sexual traits at two ages (four and nine months). Contrary to prediction, we found that post-copulatory traits senesced at a significantly slower rate than pre-copulatory traits. We also looked at whether early investment in those sexual traits affects longevity, and the interaction between sperm age (duration of sperm storage inside the male) and male age. Our results suggest that the relative senescence rate of pre- and post-copulatory sexual traits may vary among species with different mating systems and ecology.</p
CROSS-GENERATIONAL EFFECTS OF SEXUAL HARASSMENT ON FEMALE FITNESS IN THE GUPPY
No full textSexual harassment is a common outcome of sexual conflict over mating rate. A large number of studies have identified several direct costs to females of sexual harassment including energy expenditure and reduced foraging ability. However, the fitness consequences of sexual harassment for descendants have rarely been investigated. Here, we manipulated the level of sexual harassment and mating rate in two groups of female guppies,Poecilia reticulata, a live-bearing fish in which sexual conflict over mating rate is particularly pronounced. Each female was allowed to interact with three males for one day (low sexual harassment, LSH) or for eight days (high sexual harassment, HSH) during each breeding cycle throughout their life. Female lifetime fecundity did not differ between the groups, but we found a strong effect on offspring fitness. HSH females produced (1) daughters with smaller bodies and (2) sons with shorter gonopodia, which were less attractive to females and less successful in coercive matings than their LSH counterparts. Although these results may be influenced by the indirect effects of sex ratio differences between treatments, they suggest that sexual harassment and elevated mating rate can have negative cross-generational fitness effects and more profound evolutionary consequences than currently thought
Differential gene regulation in selected lines for high and low sperm production in male guppies
No full textIn species where females mate with more than one male during the same reproductive event, males typically increase the number of sperm produced to boost their fertilization share. Sperm are not limitless, however, and theory predicts that their production will come at the cost of other fitness-related traits, such as body growth or immunocompetence, although these evolutionary trade-offs are notoriously difficult to highlight. To this end, we combined artificial selection for sperm production with a transcriptome analysis using Poecilia reticulata, a fish characterized by intense sperm competition in which the number of sperm transferred during mating is the most important predictor of fertilization success, yet sperm production is highly variable among males. We compared the brain and testes transcriptome in male guppies of lines artificially selected for high and low sperm production by identifying pivotal differentially-expressed gene sets that may regulate spermatogenesis and immune function in this species. Despite the small differences in single genes’ expression, gene set enrichment analysis showed coordinated gene expression differences associated with several pathways differentially regulated in the two selection lines. High sperm production males showed an up-regulation of pathways related to immunosuppression and development of spermatozoa indicating a possible immunological cost of sperm production
Expression of pre- and postcopulatory traits under different dietary conditions in guppies
No full textSexual selection can operate both before and after mating. For males, selection will favor a suite of traits involved in mate acquisition (precopulatory) and in the competition among ejaculates to fertilize eggs (postcopulatory). Sperm competition theory predicts a trade-off between investment in pre- and postcopulatory traits, thus generating negative correlations between traits that function during both episodes of selection. However, such trade-offs can be difficult to detect because they are typically sensitive to the pool of resources available for allocation to competing functions. Although theory predicts that trade-offs will be more apparent when resources are limiting, there have been few attempts to test this prediction. In our study, we used the freshwater fish, Poecilia reticulata, to examine how both pre- and postcopulatory sexually selected traits simultaneously respond to manipulations of diet quantity by comparing behavioral, ornamental, and ejaculate traits between males assigned at random to either a ad libitum or a restricted food diet. We also explore how the relationships between traits that function during pre- and postcopulatory episodes of selection are affected by these treatments. Our results reveal that diet manipulations influenced the expression of both precopulatory (sexual behavior and ornamentation) and postcopulatory sexually selected traits (sperm viability), reinforcing the importance of resource acquisition in sexual selection. However, our data do not support the hypothesis that males trade off their allocation toward these pre- and postcopulatory sexually selected traits. Instead, changes in the expression of pre- and postcopulatory sexually selected traits appear to occur independently
Female ornamentation and directional male mate preference in the rock sparrow
No full textAlbeit there is growing evidence that males prefer to mate with ornamented females, it has been suggested that the production of costly ornaments may reduce female fecundity, hence favoring males with a preference for females with average ornamentation. In the rock sparrow, Petronia petronia, males and females possess a sexually selected patch of yellow feathers on the breast (a carotenoid-based trait). To test whether males prefer females with the largest ornament or average ornamented females, male rock sparrows were simultaneously faced with 3 conspecific females differing in breast patch size and a female house sparrow as a control. We found that the house sparrow and rock sparrow female with the smallest patch were least preferred, and males showed a clear proximity preference for the females with the above average-sized patch. Our results demonstrate that, contrary to theoretical predictions, a directional preference for female ornament was observed. Directional male preference may arise as consequence of a male's sensory bias or may be associated with indirect (genetic) benefits of choosing ornamented females, if ornament size is correlated with female genetic quality. Clearly, more work is necessary to identify the conditions under which directional preference for female ornament arise
Exploring simultaneous allocation to mating effort, sperm production and body growth in male guppies
No full textImmediate predation risk alters the relationship between potential and realised selection on male traits in the Trinidad guppy Poecilia reticulata
Full text link: Imminent predation risk affects mating behaviours in prey individuals in a multitude of ways that can theoretically impact the strength of sexual selection, as well as its operation on traits. However, empirical studies of the effects of imminent predation risk on sexual selection dynamics are still scarce. Here we explore how perceived predation affects: (1) the relationship between the opportunity for selection and the actual strength of selection on male traits; and (2) which traits contribute to male fitness and the shape of selection on these traits. We simulate two consecutive reproductive episodes, under control conditions and perceived predation risk using experimental populations of Trinidad guppies. The opportunity for selection is higher under predation risk compared to the control condition, but realised selection on traits remains unaffected. Pre- and postcopulatory traits follow complex patterns of nonlinear selection in both conditions. Differences in selection gradients deviate from predictions based on evolutionary and non-lethal effects of predation, the most notable being strong disruptive selection on courtship rate under predation risk. Our results demonstrate that sexual selection is sensitive to imminent predation risk perception and reinforce the notion that both trait-based and variance-based metrics should be employed for an informative quantification
Directional postcopulatory sexual selection is associated with female sperm storage in Trinidadian guppies
No full textFemale sperm storage (FSS) is taxonomically widespread and often associated with intense sperm competition, yet its consequences on postcopulatory sexual selection (PCSS) are poorly known. Theory predicts that FSS will reduce the strength of PCSS, because sperm characteristics favored before and after FSS may be traded-off, and opportunities for nondirectional PCSS should increase. We explored these questions in the guppy (Poecilia reticulata), by allowing females to mate multiply and by comparing the paternity pattern in two successive broods. Contrary to predictions, the variance in male fertilization success increased after FSS, driven by a change in male paternity share across broods. This change was positively associated with sperm velocity (measured before FSS) but not with the duration of FSS, indirectly suggesting that faster sperm were better in entering female storage organs, rather than in persisting within them. Other male traits, such as male size and orange color, heterozygosity, and relatedness to the female, did not influence paternity after FSS. These results indicate that processes associated with FSS tend to reinforce the strength of PCSS in guppies, rather than weaken it. Further work is necessary to test whether this pattern changes in case of more prolonged FSS
Pattern of inbreeding depression, condition dependence, and additive genetic variance in Trinidadian guppy ejaculate traits
No full textIn polyandrous species, a male's reproductive success depends on his fertilization capability and traits enhancing competitive fertilization success will be under strong, directional selection. This leads to the prediction that these traits should show stronger condition dependence and larger genetic variance than other traits subject to weaker or stabilizing selection. While empirical evidence of condition dependence in postcopulatory traits is increasing, the comparison between sexually selected and ‘control’ traits is often based on untested assumption concerning the different strength of selection acting on these traits. Furthermore, information on selection in the past is essential, as both condition dependence and genetic variance of a trait are likely to be influenced by the pattern of selection acting historically on it. Using the guppy (Poecilia reticulata), a livebearing fish with high levels of multiple paternity, we performed three independent experiments on three ejaculate quality traits, sperm number, velocity, and size, which have been previously shown to be subject to strong, intermediate, and weak directional postcopulatory selection, respectively. First, we conducted an inbreeding experiment to determine the pattern of selection in the past. Second, we used a diet restriction experiment to estimate their level of condition dependence. Third, we used a half-sib/full-sib mating design to estimate the coefficients of additive genetic variance (CVA) underlying these traits. Additionally, using a simulated predator evasion test, we showed that both inbreeding and diet restriction significantly reduced condition. According to predictions, sperm number showed higher inbreeding depression, stronger condition dependence, and larger CVA than sperm velocity and sperm size. The lack of significant genetic correlation between sperm number and velocity suggests that the former may respond to selection independently one from other ejaculate quality traits. Finally, the association between sperm number and condition suggests that this trait may mediate the genetic benefits of polyandry which have been shown in this species
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