Egg size matters for lizard sex

New findings add a surprising twist to the already complex mechanism that determines whether reptile embryos develop to be males or females. An egg-laying lizard found in the hills of southeastern Australia controls the sex of its young through the size of its eggs, suggesting that female reptiles may actively dole out yolk to fine-tune the sex ratio of their offspring, researchers linkurl:report;http://www.cell.com/current-biology/abstract/S0960-9822(09)01128-2 online today (June 4) in __Current Biology__.

Three-lined skink
Image: Sylvain Dubey

The paper "muddies the water" for everything researchers thought they knew about sex determination in lizards, linkurl:Rachel Bowden,;http://www.bio.ilstu.edu/Bowden/ an ecological physiologist at Illinois State University who was not affiliated with the study, told __The Scientist__. "It's clear that they have sex chromosomes. But it's also clear that those sex chromosomes can readily be overridden by some other factors. So, the process that leads to sex determination might be fairly plastic." Sex-determining mechanisms in reptiles can broadly be lumped into two main categories: genotypic sex determination, where offspring sex is determined by specialized sex chromosomes, and temperature-dependent sex determination, where the ambient temperature during a critical period of development controls whether an egg develops as male or female. These two mechanisms were long held to occur in different species, but researchers are now starting to discover a handful of reptiles where both sex chromosomes and environmental factors jointly shape gender. In 2002, linkurl:Rick Shine;http://www.bio.usyd.edu.au/Shinelab/shine/shine.html and his colleagues at the University of Sydney, Australia, linkurl:reported;http://www.seaturtle.org/PDF/Shine_2002_EcolLett.pdf the first such reptile, the three-lined skink lizard__ linkurl:Bassiana duperreyi,;http://www.parks.tas.gov.au/index.aspx?base=5302 __a species marked by a series of stripes running lengthways down its body. Shine's team discovered that low nest temperatures could reverse the genotypic sex of __B. duperreyi__ embryos, such that genotypically-female eggs with two X chromosomes could mature into phenotypic males with only male reproductive organs. What's more, the researchers noticed a "weird correlation" -- eggs that produced daughters were typically larger than those that produced sons. "We confidently assumed that couldn't be a causative effect," Shine told __The Scientist__, and the researchers didn't explore the issue further at the time. Seven years on, however, Shine's group has cracked the egg size question, and shown that egg size can, indeed, determine offspring sex. First, Shine's team weighed more than 800 eggs from 130 clutches and found that son-producing eggs were consistently a few fractions of a gram lighter than daughter-producing eggs. The researchers then incubated eggs at cool and warm temperatures and fiddled with yolk levels. At 22°C, adding or removing yolk had no effect on the nearly-even offspring sex ratios. But at 16°C, "to our astonishment, the eggs changed sex," said Shine: 88% of yolk-removed eggs and 7% of yolk-added eggs developed as males, they found. Swapping silicone for yolk, however, had no effect, indicating that some component of the yolk sac, possibly sex hormones, pushes larger eggs phenotypically toward femininity. "That's pretty intriguing evidence that it's a yolk component -- it's not just having a bigger egg -- that drives this process," said Bowden. "But we don't know what that component of yolk is." Bowden said this paper should redouble people's efforts to investigate the egg's molecular componentry. How did this three-pronged approach to sex determination -- involving a combination of sex chromosomes, nest temperatures, and egg size -- arise? Probably in a stepwise fashion, speculated Shine. He linkurl:previously showed;http://www.nature.com/nature/journal/v378/n6556/abs/378451a0.html that __B. duperreyi__ daughters fare less well than sons in cold nests. Thus, selection may have favored sons at the cooler end of the thermal spectrum, allowing temperature-dependent sex determination to kick in and supersede the sex chromosomes. But if nest temperatures dropped so low that most offspring were born as sons, there would be a large reproductive advantage to having daughters, even somewhat sickly ones, and so egg size-dependent sex determination could have evolved to move sex ratios back closer to 50:50. "It's really just a succession of selective forces," Shine said. Bowden called this model "a really interesting approach and a novel way of thinking about what's turning out to be a very complicated system." The elaborate sex determining system seen in __B. duperreyi__ is probably also not unique to these slinky skinks, Shine noted. "My guess is that we're going to discover that this is incredibly widespread" among reptiles.
**__Related stories:__***linkurl:Genetic basis of XX males discovered;http://www.the-scientist.com/news/display/25087/
[16th October 2006]*linkurl:Platypus has 10 sex chromosomes;http://www.the-scientist.com/article/display/22471/
[28th October 2004]*linkurl:Sex determination in fish;http://www.the-scientist.com/article/display/20398/
[21st May 2002]