Is it a Boy or a Girl? For Some Reptiles, It May Depend on Egg Size

Whether an animal develops into a male or female is in most cases determined by specialized sex chromosomes. In most mammals, for example, males have an X chromosome and a Y chromosome, and females have two X chromosomes. Because they can give either an X or a Y chromosome, male mammals determine the sex of their offspring. For some reptiles, such as turtles, ambient environmental temperature has an impact on sex determination. Research indicates that turtle eggs in cooler nests tend to develop into males and eggs in warmer nests tend to develop into females.

Researchers in Australia recently discovered an interesting twist to sex determination in one type of reptile. Studies of the lizard Bassiana duperreyi showed that when the lizard’s eggs were exposed to extreme low temperatures, the sex of the embryos switched, regardless of chromosomes, sometimes even resulting in XX males and XY females. Further research showed that large eggs tended to result in female offspring and smaller eggs tended to result in male offspring.

The scientists initially gave little thought to this discovery, assuming that the correlation between egg size and sex was an anomaly. However, further research indicated that egg size did indeed have an impact on sex. The scientists were surprised to find that when yolk was removed from an egg, the embryo had a tendency to switch to a male, even when the embryo had female chromosomes. Conversely, when yolk was added to an egg, the embryo had a tendency to switch to a female. According to Dr. Richard Shine, one the Australian scientists involved with the study, the researchers were “gobsmacked” by these results and look forward to the opportunity to further research the mechanics behind this interesting phenomenon.

The results of the scientists’ research was published in the June 4, 2009 online edition of the journal Current Biology. Scientists who contributed to the paper included Rajkumar S. Radder, David A. Pike and Richard Shine, all from the University of Sydney, and Alexander E. Quinn from the University of Canberra.

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Novel Brain Region for Mammalian Neurogenesis

Adult neurogenesis is a mysterious phenomenon. Previously, scientists thought that adult neurogenesis, or the birth of new neurons, occurred most often in lower vertebrates such as frogs or fish. In mammals such as mice and rats, adult neurogenesis was thought to be rare, and restricted to two particular brain regions.

However, two groups of scientists at the University of Turin in Italy have recently documented neurogenesis in adult rabbits. Surprisingly, the birth of progenitor cells appears to occur in the cerebellum, a brain region previously thought to lack cell turnover in adulthood.

In mammals, progenitor cells typically originate from remnants of embryonic cell layers, such as the ventricular zone or the olfactory bulb. The progenitor cells identified in the current study are generated from neuronal progenitor cells in the functional tissue of the brain, suggesting that these cells might have great potential for the repair of neural damage in adult tissue.

While rabbits do live longer than other model organisms for adult neurogenesis, and thus might have a different neural development profile, this work gives new hope to the study of adult neurogenesis in humans.

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Chameleon Spends Most of Its Life in Its Egg

“Life is short” isn’t just a witty saying for a certain species of chameleons that live in southwestern Madagascar. For the Labord’s chameleon (Furcifer labordi), life really is short. And, to make it even more interesting, the majority of its shortened life is spent within the confines of an egg. After hatching from its egg, the chameleon only lives another four or five months before its life is over.

An Accidental Discovery

The unusual natural history of these chameleons was discovered by researchers by accident. Kristopher Karsten, a graduate student based at Oklahoma State University, arrived late in the season. Although he expected to find many recently-hatched juvenile chameleons, he didn’t find any. Then, several months later in February, he discovered something even more unusual–the carcasses of many Labord’s chameleons, apparently dead from natural causes.

A male Labord’s chameleon. (Credit: Ken Preston-Mafham/PREMAPHOTOS/naturepl.com)

At the time of Karsten’s observations, not much was known about the life history of Labord’s chameleons. Since his initial observations, more than 400 individuals have been studied over a period of five field seasons and scientists now know much more about the species’ remarkable life history.

Shortest Lifespan, Fastest Growth Rate

Because of its short life span outside of the egg, it is imperative that the chameleon matures quickly–and that it does. Labord’s chameleons have the fastest growth rates of any tetrapod species. After hatching during Madagascar’s wet season in November, the chameleons grow at a rate of up to 2.6 mm (0.1 inches) per day. In fact, in less than two months, the chameleons increase in body size by 300-400 percent. To put that another way, if a human grew at the same rate, an infant that was 51 cm (20 inches) at birth would grow to a height of 1.5 meters (five ft.) over a period of only two months.

After reaching maturity, the chameleon population reproduces. Females dig burrows in the desert sand and bury their eggs about 138 mm (5.4 inches) below the surface. Following reproduction, all of the adult chameleons die. Meanwhile, the developing chameleons remain in their eggs underground for eight to nine months, where they wait out the dry season.

A female Labord’s chameleon. (Credit: PREMAPHOTOS/naturepl.com)

Ecological Constraints

So what accounts for such an unusual life history? Scientists point to several factors that may have led the species to evolve such a short lifespan. One factor is size–Labord’s chameleons are the smallest of its genus. Perhaps due to their size, the chameleons are a favorite snack of desert predators such as birds and snakes. A second factor is climate. The southwestern Madagascar desert climate where the chameleons live is harsh and unpredictable. In addition, the rainy season is short, which further limits the amount of time the chameleons have to grow and reproduce.

Questionable Future

Though scientists have learned a lot about Labord’s chameleons, many questions remain. While the short lifespan of the Labords chameleon lets it successfully survive the constraints of its ecology, scientists wonder how long-term changes in the climate–or even a short-term event like a longer-than-normal dry season–might affect the species.

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