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Evolutionary Diversity and Turn-Over of Sex Determination in Teleost Fishes

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Evolutionary Diversity and Turn-Over of Sex Determination in Teleost Fishes Review Article Sex Dev 2009;3:60–67 Received: July 21, 2008 DOI: 10.1159/000223071 Accepted: August 28, 2008 Evolutionary Diversity and Turn-Over of Sex Determination in Teleost Fishes a b J.E. Mank J.C. Avise a b Department of Zoology, University of Oxford, Oxford , UK; Department of Ecology and Evolutionary Biology, University of California, Irvine , Calif., USA Key Words Sexual reproduction in its most basic form does not Environmental sex determination ؒ Hermaphroditism ؒ require distinct sexes, but rather simply the union of 2 Sex chromosomes gametes of any form. However, sexual reproduction often leads to anisogamy, or size-differentiated male and fe- male gametes, and while anisogamy in theory involves Abstract strictly the gametes and the gonads that produce them, Sex determination, due to the obvious association with re- the evolutionary process that begins with gametic dimor- production and Darwinian fitness, has been traditionally as- phism has extended consequences in many animals. An- sumed to be a relatively conserved trait. However, research isogamy triggers an evolutionary cascade that extends on teleost fishes has shown that this need not be the case, beyond the gonad to the soma, as circulating sex hor- as these animals display a remarkable diversity in the ways mones involved in gonadal and gametic differentiation that they determine sex. These different mechanisms, which also influence somatic tissues and behaviors [Zauner et include constitutive genetic mechanisms on sex chromo- al., 2003; Ketterson et al., 2005; Mank, 2007; McGlothlin somes, polygenic constitutive mechanisms, environmental and Ketterson, 2008]. Therefore, whether an individual influences, hermaphroditism, and unisexuality have each animal develops as a male or female greatly influences its originated numerous independent times in the teleosts. The life experience, as sex can shape the way an individual evolutionary lability of sex determination, and the corre- lives, how it acts [Nottebohm and Arnold, 1976; Datta et sponding rapid rate of turn-over among different modes, al., 2008], and ultimately how it dies [Vandenbroeck et al., makes the teleost clade an excellent model with which to 2003; Ivakine et al., 2005; Naugler et al., 2007]. Males and test theories regarding the evolution of sex determining ad- females of the same species may also experience different aptations. Much of the plasticity in sex determination likely evolutionary pressures [Mank, 2009] and often corre- results from the dynamic teleost genome, and recent ad- spondingly display vastly different phenotypes [An- vances in fish genetics and genomics have revealed the role dersson, 1994]. Sex is therefore an important biological of gene and genome duplication in fostering emergence and evolutionary component, and so it is hardly surpris- and turn-over of sex determining mechanisms. ing, given the importance of sexual reproduction in a Copyright © 2009 S. Karger AG, Basel Darwinian fitness context, that elements of sex deter- mining pathways are conserved across the animal king- dom [Volff et al., 2003; Ferguson-Smith, 2007]. That said, © 2009 S. Karger AG, Basel Judith E. Mank 1661–5425/09/0033–0060$26.00/0 University of Oxford Fax +41 61 306 12 34 Department of Zoology E-Mail [email protected] Accessible online at: South Parks Road, Oxford, OX1 3PS (UK) www.karger.com www.karger.com/sxd E-Mail [email protected] the actual mechanism by which this pathway is triggered Types of Sex Determination in Fish varies widely and can be a function of inheritance, envi- ronment, or both. Sex is ultimately determined via a biochemical cas- In scientific thinking and research, there is a bias to- cade that initiates the formation of either the male or the wards sex chromosomes, which is primarily due to the female gonad. We concern ourselves here with the ulti- fact that most of the major animal models, including hu- mate factors that initiate that cascade, typically referred mans, possess them. Sex chromosomes arose once in the to as sex determinants, rather than the proximate genes ancestor of the therian mammals, and all modern theri- in the pathway itself. It is useful to delineate these deter- ans possess orthologs from this event [Lahn and Page, minants by whether they are internal (inherited and con- 1999; Waters et al., 2005; Potrzebowski et al., 2008]. Sex stitutive) or external (environmental or inducible) to the chromosomes arose independently in the avian ancestor, organism. and all modern bird sex chromosomes are descended from this ancestral event [Fridolfsson et al., 1998; Mank Sex Chromosomes and Other Inherited Mechanisms and Ellegren, 2007]. Some of the most popular model in- Internal factors determine sex at conception by inher- vertebrates, namely Caenorhabditis and Drosophila, also itance. Sex chromosomes are the most commonly identi- possess sex chromosomes [ C. elegans Genome Sequenc- fied carriers of constitutive genetic sex determination ing Consortium, 1998; Drosophila 12 Genomes Consor- and derive originally from autosomes that contain a clus- tium, 2007]. All this produces a misconception that sex ter of closely-linked sex determining genes [Ohno, 1967; chromosomes are the predominant mechanism by which Charlesworth and Charlesworth, 1978] and which are of- sex is conferred in animals. ten called proto-sex chromosomes. There is selection to In fact, sex can be determined in a variety of ways, and suppress recombination between the male and female re- perhaps no group of animals illustrates this more clearly gions of the proto-sex chromosomes, and this yields a sex than the teleost fishes. Since its Triassic origin, this clade chromosomal inheritance pattern and neo-sex chromo- has evolved and re-evolved a remarkable array of sex de- somes [Charlesworth et al., 2005]. These neo-sex chro- termining mechanisms, including constitutive genes mosomes often lack morphological differences at this linked to sex chromosomes, multi-locus autosomal trig- point, and are homomorphic, or visually similar, in chro- gers, environmental factors including temperature, food, mosome stains and squashes, which are generally the and nest availability, as well as demographic and social most common genomic scans. Subsequent extensions of influences [Devlin and Nagahama, 2002; Mank et al., the region of recombination suppression, accompanied 2006]. Furthermore, sex need not be a life-long condition by chromosomal inversions on one sex chromosome, will in fish, as many groups are simultaneously or sequen- lead to degradation of the sex-limited chromosome tially hermaphroditic [see Avise and Mank, this issue]. [Charlesworth, 1991; Vicoso and Charlesworth, 2006] Several clades have even abandoned sex entirely and and the emergence of heteromorphic, or visually distinct, evolved unisexual reproductive modes [Avise, 2008]. sex chromosomes [Graves, 1998; Charlesworth, 1996). Clearly for this group of animals, sex chromosomes are Sex chromosomes do not, by themselves, determine sex, just one of many different options for determining who but rather are an epiphenomenon resulting from the con- will be female and who will be male. stitutively expressed sex determining genes that they This wealth of diversity brings with it several ques- contain. tions that we address in this review. Here we summarize Sex chromosomes may take either of 2 inheritance what is known about sex determining mechanisms in fish patterns ( fig. 1 ). Male heterogamety (where females are and then attempt to reconcile this with current genomic XX and males XY) and female heterogamety (where fe- and evolutionary theory. males are ZW and males ZZ) have both evolved multiple Examples will be given for different kinds of sex deter- independent times in fish [Devlin and Nagahama, 2002; mination in teleosts, but for an exhaustive catalogue of Woram et al., 2003; Mank et al., 2006]. Sex chromosomes sex determining mechanisms, we refer the reader to the have been observed in about 10% of surveyed species work of Devlin and Nagahama [2002]. Similarly, we have [Devlin and Nagahama, 2002], though this is a gross un- previously addressed the broad-scale comparative phylo- derestimate as proto- and neo-sex chromosomes lack dis- genetics of sex determination in teleosts [Mank et al., tinguishable karyotypic differences between the large 2006] and so refer the reader there for a comparative evo- major (X or Z) and small sex-limited minor (Y or W) sex lutionary analysis across the clade. chromosomes. When nascent sex chromosomes are ac- The Evolution of Sex Determination Sex Dev 2009;3:60–67 61 fail to reproduce. Males will then have to compete for ac- a Male heterogamety b Female heterogamety cess to females, and this competition may explain the Father Father presence of sex chromosomes in lineages with sexually selected male traits and large skews in male mating suc- XY ZZ cess, such as the Xiphophorus genus [Nanda et al., 2000; Kingston et al., 2003]. XXXXY ZZZZZ Some species have internal constitutive genetic sex de- termination without sex chromosomes. This can happen Mother Mother when a single gene initiates the pathway, but genetic re- XXXXY WZWZW combination in the area has not yet been suppressed, and thus the sex chromosomes have not diverged. These are analogous to the fully-fledged sex chromosomes de- scribed above, yet are cytologically cryptic. Sex can also Fig. 1. The inheritance of the minor sex chromosome (Y or W) is be determined by polygenic internal factors, such as the correlated with sex. Because the odds of inheriting the minor sex allelic combination of unlinked genes seen in Dicentrar- chromosome are even in both cases, the resulting sex ratio is bal- chus labrax, the European sea bass [Vandeputte et al., anced in male heterogametic ( a ) and female heterogametic ( b ) 2007], or the combination of competing female- and species. In both panels, female offspring are shown in red, male male-heterogametic mechanisms that has been shown in blue. for some cichlids [Cnaani et al., 2007]. E x t e r n a l I n f l u e n c e s counted for, sex chromosomes may be present in up to Many species of fish delay sex determination until 50% of teleost species [Arkhipchuk, 1995].
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