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Male Sexual Ornament Size Is Positively Associated With BMC Evolutionary Biology BioMed Central Research article Open Access Male sexual ornament size is positively associated with reproductive morphology and enhanced fertility in the stalk-eyed fly Teleopsis dalmanni David W Rogers1,2, Matthew Denniff1,3, Tracey Chapman1,4, Kevin Fowler1 and Andrew Pomiankowski*1,5,6 Address: 1The Galton Laboratory, Research Department of Genetics, Evolution and Environment, University College London, 4 Stephenson Way, London, NW1 2HE, UK, 2Division of Cell and Molecular Biology, Imperial College London, Imperial College Road, London, SW7 2AZ, UK, 3Department of Genetics, University of Leicester, University Road, Leicester, LE1 7RH, UK, 4School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK, 5Collegium Budapest, Szentháromság utca 2, H-1014 Budapest, Hungary and 6CoMPLEX, University College London, Gower Street, London, WC1E 6BT, UK Email: David W Rogers - [email protected]; Matthew Denniff - [email protected]; Tracey Chapman - [email protected]; Kevin Fowler - [email protected]; Andrew Pomiankowski* - [email protected] * Corresponding author Published: 18 August 2008 Received: 25 February 2008 Accepted: 18 August 2008 BMC Evolutionary Biology 2008, 8:236 doi:10.1186/1471-2148-8-236 This article is available from: http://www.biomedcentral.com/1471-2148/8/236 © 2008 Rogers et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: Exaggerated male ornaments and displays often evolve in species where males only provide females with ejaculates during reproduction. Although "good genes" arguments are typically invoked to explain this phenomenon, a simpler alternative is possible if variation in male reproductive quality (e.g. sperm number, ejaculate content, mating rate) is an important determinant of female reproductive success. The "phenotype- linked fertility hypothesis" states that female preference for male ornaments or displays has been selected to ensure higher levels of fertility and has driven the evolution of exaggerated male traits. Females of the stalk-eyed fly Teleopsis dalmanni must mate frequently to maintain high levels of fertility and prefer to mate with males exhibiting large eyespan, a condition-dependent sexual ornament. If eyespan indicates male reproductive quality, females could directly increase their reproductive success by mating with males with large eyespan. Here we investigate whether male eyespan indicates accessory gland and testis length, and then ask whether mating with large eyespan males affects female fertility. Results: Male eyespan was a better predictor of two key male reproductive traits – accessory gland and testis length – than was body size alone. This positive relationship held true over three levels of increasing environmental stress during the maturation of the adult accessory glands and testes. Furthermore, females housed with a large eyespan male exhibited higher levels of fertility than those with small eyespan males. Conclusion: Male eyespan in stalk-eyed flies is subject to strong directional mate preference and is a reliable indicator of male reproductive quality – both because males with larger eyespan have bigger accessory glands and testes, and also as they confer higher fertility on females. Fertility enhancement may have arisen because males with larger eyespan mated more often and/or because they transferred more sperm or other substances per ejaculate. The need to ensure high levels of fertility could thus have been an important selective force in the coevolution of female preference and male eyespan in stalk-eyed flies. Our results support the phenotype-linked fertility hypothesis and suggest that it might be of general importance in explaining the evolution of exaggerated male ornaments and displays in species where males only provide females with ejaculates during reproduction. Page 1 of 8 (page number not for citation purposes) BMC Evolutionary Biology 2008, 8:236 http://www.biomedcentral.com/1471-2148/8/236 Background eyespan is a highly condition-dependent trait which is Female mate preferences can evolve for exaggerated male sensitive to environmental stress during larval develop- traits that are indicators of male quality [1]. In resource- ment [21-23], external morphology is fixed at eclosion free mating systems in which males only provide females [24]. This seems likely to limit the effectiveness of eyespan with ejaculates, it is often assumed that choosy females as a signal of accessory gland and testis size as these traits benefit through indirect genetic benefits to their offspring are strongly influenced by environmental stress during [2]. However, ejaculates vary in the amount of accessory their maturation in adult flies [13]. Consequently there is substances as well as the number and quality of sperm a compelling need to explicitly investigate whether male transferred, and these constituents are important determi- eyespan predicts the growth and size at maturity of the nants of female fertility, fecundity and longevity [3]. Con- internal reproductive organs, and whether it influences sequently, if male ornamentation also signals ejaculate female fertility. quality, this is likely to be an additional force favouring the evolution of female mate preferences. The possibility that fertility assurance is a major selective force in the evolution of female mate preferences for exag- In this paper, we investigate whether male ornamentation gerated male sexual ornaments has been discussed most signals reproductive quality in the stalk-eyed fly Teleopsis extensively under the heading of the "phenotype-linked dalmanni (synonym Cyrtodiopsis dalmanni [4]), in particu- fertility hypothesis". This hypothesis originally linked fer- lar, the ability of males to confer fertility. There is evidence tility assurance to extra-pair matings in socially monoga- that variation in fertility is an important component of mous birds [25,26] but has since been generalized to all female fitness in T. dalmanni. Multiple copulations are cases where sexual ornaments covary with male fertility required by females to achieve and maintain high fertility traits [27,28]. More recently, the idea has been framed [5]. This is probably because of the small size of the male within the handicap principle, with the implication that spermatophore [6,7] and the small number of sperm the covariance between male ornaments and fertilizing stored following a single copulation (approximately 65 efficiency arises because both are condition-dependent [8]). This effect is compounded by X-linked meiotic drive, traits [29]. So males in good condition not only develop which is found in 13–17% of field collected males [9]. more attractive ornaments but also make greater invest- Meiotic drive impairs the development of Y-bearing ment in male fertility traits. sperm, causing lowered fertility in females mated to drive males [10,11]. Consistent with these findings, less than Empirical evidence for the hypothesis is ambiguous 40% of eggs laid by field-caught females failed to hatch, [27,30]. For example, studies using commercial strains or though the exact cause of this has yet to be established commercial/wild hybrids of domestic fowl have largely [12]. supported the hypothesis [29,31], whereas no relation- ship between male comb size and testis size was found in Male reproductive quality is associated with the size of the a captive population of Red Jungle fowl, the conspecific accessory glands and testes. These internal reproductive ancestor of domestic fowl [32]. The strongest support for organs are very small at eclosion, and then dramatically the phenotype-linked fertility hypothesis comes from increase in size, allowing males to attain sexual maturity Wagner & Harper's [33] study of field crickets. Male chirp three to four weeks later [13]. Accessory gland growth rate rate is positively correlated with the number of sperm is positively correlated with the time required to reach sex- transferred and females mated to males with higher chirp ual maturity [13] while the size of the mature accessory rates exhibited higher lifetime fertility. However, Wagner glands is both phenotypically [13,14] and genetically [15] & Harper [33] may have exaggerated the importance of correlated with male mating frequency. The ability of a this finding as females were restricted to a single mating; male to mate multiply is likely to be an important contrib- given the opportunity, female field crickets normally utor to female fertility, as matings are concentrated in the remate at high frequency [34]. Furthermore, positive periods shortly before dusk and after dawn when single results were only observed in females fed a restricted diet; males mate with several females [16]. Testis size is posi- females fed a standard diet received no fertility benefits tively correlated with the number of sperm stored in a from males with high chirp rates. female's spermathecae following copulation [17] and thus is also likely to affect female fertility. In this paper we consider three fundamental requirements for the phenotype-linked fertility hypothesis to be sup- A number of experiments have established that females ported. First, the male ornament or display must be sub- prefer to roost and copulate with
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