Nicole Valenzuela

Sex Determination: Why So Many Ways of Doing It?

Sex is universal amongst most eukaryotes, yet a remarkable diversity of sex determining mechanisms exists. We review our current understanding of how and why sex determination evolves in animals and plants.

Are all sex chromosomes created equal

Three principal types of chromosomal sex determination are found in nature: male heterogamety (XY systems, as in mammals), female heterogamety (ZW systems, as in birds), and haploid phase determination (UV systems, as in some algae and bryophytes). Although these systems share many common features, there are important biological differences between them that have broad evolutionary and genomic implications. Here we combine theoretical predictions with empirical observations to discuss how differences in selection, genetic properties and transmission uniquely shape each system. We elucidate how the differences among these systems can be exploited to gain insights about general evolutionary processes, genome structure, and gene expression. We suggest directions for research that will greatly increase our general understanding of the forces driving sex-chromosome evolution in diverse organisms.

Sex Chromosome Evolution in Amniotes: Applications for Bacterial Artificial Chromosome Libraries

Variability among sex chromosome pairs in amniotes denotes a dynamic history. Since amniotes diverged from a common ancestor, their sex chromosome pairs and, more broadly, sex-determining mechanisms have changed reversibly and frequently. These changes have been studied and characterized through the use of many tools and experimental approaches but perhaps most effectively through applications for bacterial artificial chromosome (BAC) libraries. Individual BAC clones carry 100–200 kb of sequence from one individual of a target species that can be isolated by screening, mapped onto karyotypes, and sequenced. With these techniques, researchers have identified differences and similarities in sex chromosome content and organization across amniotes and have addressed hypotheses regarding the frequency and direction of past changes. Here, we review studies of sex chromosome evolution in amniotes and the ways in which the field of research has been affected by the advent of BAC libraries.

Estimating population structure under nonequilibrium conditions in a conservation context: continent‐wide population genetics of the giant Amazon river turtle, Podocnemis expansa (Chelonia; Podocnemididae)

Giant Amazon river turtles, Podocnemis expansa, are indigenous to the Amazon, Orinoco, and Essequibo River basins, and are distributed across nearly the entire width of the South American continent. Although once common, their large size, high fecundity, and gregarious nesting, made P. expansa especially vulnerable to over‐harvesting for eggs and meat. Populations have been severely reduced or extirpated in many areas throughout its range, and the species is now regulated under Appendix II of the Convention on International Trade in Endangered Species. Here, we analyse data from mitochondrial DNA sequence and multiple nuclear microsatellite markers with an array of complementary analytical methods. Results show that concordance from multiple data sets and analyses can provide a strong signal of population genetic structure that can be used to guide management. The general lack of phylogeographic structure but large differences in allele and haplotype frequencies among river basins is consistent with fragmented populations and female natal‐river homing. Overall, the DNA data show that P. expansa populations lack a long history of genetic differentiation, but that each major tributary currently forms a semi‐isolated reproductive population and should be managed accordingly.

Maternal Effects on Life-History Traits in the Amazonian Giant River Turtle Podocnemis expansa

-Energy allocation to eggs and nest site selection by females can affect life-history variables such as offspring size, offspring number, developmental rate, survivorship, growth rate, and performance in oviparous reptiles. Nest site selection can affect offspring phenotype by altering incubation conditions. I present evidence of a positive effect of female size on clutch size, egg mass, and nest depth through the study of trackways left by female river turtles, Podocnemis expansa, on their nesting beaches. Larger females laid larger clutches composed of larger eggs, which were buried deeper than clutches laid by smaller females. The data suggest that P. expansa does not conform to optimal propagule size models. Neither egg size nor clutch size reached a plateau as female size increased. Females seem to allocate the extra energy (in absolute terms) gained allometrically with increasing size and age to both number and size of eggs. There was no evidence of a trade-off between egg size and number after removing the effect of female size. Larger eggs produced larger hatchlings that survived better but grew less than individuals of smaller initial size during the first two months of life, under unlimited food conditions. I suggest that fitness of female P. expansa increases by producing larger eggs because of the advantage that larger hatchlings have in survival. Deeper nests experience cooler temperatures and tend to produce a higher percentage of males than more superficial nests. Therefore, there is a potential for important effects of nest depth on sex ratios produced by different sized females within the population and possibly by single females throughout their lifetime. Constant temperature in artificial incubation experiments had an effect on the size of individuals at hatching, but differences vanished by the second month of age via the greater growth rate shown by individuals of smaller initial size. urnal of Herpetology, Vol. 35, No. 3, pp. 368-378, 2001 yright 2001 Society for the Study of Amphibians and Reptiles aternal Effects on Life-Histo y Trai s in the Amazonian Giant River rtle Podocnemis exp nsa Life-history variables such as offspring size, offspring number, developmental rate, survivorship, growth rate, and performance may be influenced by maternal factors such as energy allocation and nest site selection in oviparous reptiles. Because those parameters affect offspring fitness, females could maximize their own fitness by optimizing that of their offspring (Brockelman, 1975). On one hand, species with larger clutches and no parental care are expect1 Present Address: Department of Zoology and Genetics, Iowa State University, Ames, Iowa 50011, USA; E-mail: nvalenzu@iastate.edu. istory variables such as offspring size, ing number, developmental rate, survivor, rowth rate, and performance may be inced by maternal factors uch as en rgy alio and nest site sel ction in oviparous rep. ecause those parameters affect offspring ed to show the patterns of energy allocation predicted by optimality models (Smith and Fretwell, 1974), which may involve compromises between longevity and fertility, and trade-offs between offspring size and offspring number (Roff, 1992; Stearns, 1992; Bulmer, 1994; Sikes, 1998 and references therein). On the other hand, nest site selection can alter incubation conditions experienced by the offspring that can affect their phenotype (Shine and Harlow, 1996). Incubation conditions such as temperature vary with nest substrate characteristics, sun and wind exposure, and nest depth (Souza and Vogt, 1994; Janzen, 1994; Shine and Harlow, 1996). Incubation temperature has profound effects on s o the pa terns of energy allocation preoptimality models (Smith and Frel, 974), which may involve compromi es longevity and fert lity, and trade-offs offspring size and o fspring number 368 This content downloaded from 157.55.39.17 on Wed, 31 Aug 2016 04:39:41 UTC All use subject to http://about.jstor.org/terms LIFE HISTORY OF AMAZONIAN GIANT TURTLES reptiles, particularly for species with temperature-dependent sex determination (TSD). Females allocate energy to eggs and choose nest site locations within each reproductive season, and it has been suggested that certain combinations of both parameters (nest-site choice dependent on egg size) could be adaptive in turtle species (Roosenburg, 1996). Here, I report on the effects of female size of a freshwater turtle species (Podocnemis expansa) on egg size, egg number, and an aspect of nest location, namely, nest depth. Podocnemis expansa is a large species inhabiting the Amazon and Orinoco basins in South America. The size of adult females ranges between 50 and 80 cm carapace length (Hildebrand et al., 1997). Descriptions of the nesting behavior in P expansa report that females excavate a nest hole such that their head is level with the surface of the beach while ovipositing (Mosqueira-Manso, 1945; Ramirez, 1956; Vanzolini, 1967). Therefore, large females should dig deeper holes than smaller females. Ramirez (1956) suggested a positive relationship between female size and nest depth, but no data have been presented to test this prediction. Podocnemis expansa exhibits temperature-dependent sex determination (TSD), such that males are produced at low incubation temperatures and females at high temperatures (Alho et al., 1985; Valenzuela et al., 1997). Females nest in sandbars where temperature varies considerably with depth in the nesting substrate (Valenzuela, 1999). An experimental study in the same nesting area revealed a significant effect of the depth of artificial nests on offspring sex ratio (Valenzuela, 1999). In general, the deeper the nest the cooler and less variable the incubation conditions, which tended to produce more males (Valenzuela, 1999). The experimental effect of nest depth on sex ratio would be of great importance if similar nest depth variation were present in the wild. Particularly, if nest depth is correlated with female size within the population, or within a given female as her size increases with age, then the population sex ratio or a females lifetime sex-ratio production could be affected. These considerations are especially important given the endangered status of P ex-

Tree of Sex: A database of sexual systems

The vast majority of eukaryotic organisms reproduce sexually, yet the nature of the sexual system and the mechanism of sex determination often vary remarkably, even among closely related species. Some species of animals and plants change sex across their lifespan, some contain hermaphrodites as well as males and females, some determine sex with highly differentiated chromosomes, while others determine sex according to their environment. Testing evolutionary hypotheses regarding the causes and consequences of this diversity requires interspecific data placed in a phylogenetic context. Such comparative studies have been hampered by the lack of accessible data listing sexual systems and sex determination mechanisms across the eukaryotic tree of life. Here, we describe a database developed to facilitate access to sexual system and sex chromosome information, with data on sexual systems from 11,038 plant, 705 fish, 173 amphibian, 593 non-avian reptilian, 195 avian, 479 mammalian, and 11,556 invertebrate species.

The lesser known challenge of climate change: thermal variance and sex-reversal in vertebrates with temperature-dependent sex determination.

Climate change is expected to disrupt biological systems. Particularly susceptible are species with temperature-dependent sex determination (TSD), as in many reptiles. While the potentially devastating effect of rising mean temperatures on sex ratios in TSD species is appreciated, the consequences of increased thermal variance predicted to accompany climate change remain obscure. Surprisingly, no study has tested if the effect of thermal variance around high-temperatures (which are particularly relevant given climate change predictions) has the same or opposite effects as around lower temperatures. Here we show that sex ratios of the painted turtle (Chrysemys picta) were reversed as fluctuations increased around low and high unisexual mean-temperatures. Unexpectedly, the developmental and sexual responses around female-producing temperatures were decoupled in a more complex manner than around male-producing values. Our novel observations are not fully explained by existing ecological models of development and sex determination, and provide strong evidence that thermal fluctuations are critical for shaping the biological outcomes of climate change.

An XX/XY sex microchromosome system in a freshwater turtle, Chelodina longicollis (Testudines: Chelidae) with genetic sex determination.

Heteromorphic sex chromosomes are rare in turtles, having been described in only four species. Like many turtle species, the Australian freshwater turtle Chelodina longicollis has genetic sex determination, but no distinguishable (heteromorphic) sex chromosomes were identified in a previous karyotyping study. We used comparative genomic hybridization (CGH) to show that C. longicollis has an XX/XY system of chromosomal sex determination, involving a pair of microchromosomes. C-banding and reverse fluorescent staining also distinguished microchromosomes with different banding patterns in males and females in ∼70% cells examined. GTG-banding did not reveal any heteromorphic chromosomes, and no replication asynchrony on the X or Y microchromosomes was observed using replication banding. We conclude that there is a very small sequence difference between X and Y chromosomes in this species, a difference that is consistently detectable only by high-resolution molecular cytogenetic techniques, such as CGH. This is the first time a pair of microchromosomes has been identified as the sex chromosomes in a turtle species.

Geometric Morphometric Sex Estimation for Hatchling Turtles: A Powerful Alternative for Detecting Subtle Sexual Shape Dimorphism

Abstract Identifying sex of hatchling turtles is difficult because juveniles are not obviously externally dimorphic, and current techniques to identify sex are often logistically unfeasible for field studies. We demonstrate a widely applicable and inexpensive alternative to detect subtle but significant sexual dimorphism in hatchlings, using landmark-based geometric morphometric methods. With this approach, carapace landmarks were digitized from photographs of each hatchling, and shape variables were generated after variation in size, location and orientation were eliminated. These variables were then analyzed for sexual dimorphism, and used in discriminant function analysis to estimate sex of each hatchling. Using this approach on two species (Chrysemys picta and Podocnemis expansa), we found this method had high accuracy in assigning sex when compared with true sex (98% and 90%, respectively), and cross-validation revealed a correct classification rate of 85%. These correct classification rates were considerably higher than those found on the same species using linear distance measurements as data. We also explored two alternative statistical approaches for assessing sex (K-means clustering and multiple logistic regression) and found that these alternative approaches were accurate only 61% and 78% of the time, respectively, in C. picta and 69% and 77% of the time in P. expansa. These findings are similar to classification rates found for turtle species using approaches based on linear distance measurements. We also found that the observed sexual dimorphism differed between the two species. In P. expansa, males displayed relatively more expansion of the central region of the carapace relative to females, whereas in C. picta this pattern was reversed. We conclude that discriminant analysis of morphology quantified using geometric morphometrics provides researchers with a powerful tool to discriminate between male and female hatchling turtles.

Y Fuse? Sex Chromosome Fusions in Fishes and Reptiles

Chromosomal fusion plays a recurring role in the evolution of adaptations and reproductive isolation among species, yet little is known of the evolutionary drivers of chromosomal fusions. Because sex chromosomes (X and Y in male heterogametic systems, Z and W in female heterogametic systems) differ in their selective, mutational, and demographic environments, those differences provide a unique opportunity to dissect the evolutionary forces that drive chromosomal fusions. We estimate the rate at which fusions between sex chromosomes and autosomes become established across the phylogenies of both fishes and squamate reptiles. Both the incidence among extant species and the establishment rate of Y-autosome fusions is much higher than for X-autosome, Z-autosome, or W-autosome fusions. Using population genetic models, we show that this pattern cannot be reconciled with many standard explanations for the spread of fusions. In particular, direct selection acting on fusions or sexually antagonistic selection cannot, on their own, account for the predominance of Y-autosome fusions. The most plausible explanation for the observed data seems to be (a) that fusions are slightly deleterious, and (b) that the mutation rate is male-biased or the reproductive sex ratio is female-biased. We identify other combinations of evolutionary forces that might in principle account for the data although they appear less likely. Our results shed light on the processes that drive structural changes throughout the genome.