Tuatara do things a little differently to other reptiles when it comes to sex determination – not because they have temperature-dependent sex determination (thats common to lots of reptiles), but because their pattern of temperature-dependent sex determination (or TSD) is different from most other reptiles. For tuatara, incubating eggs at higher temperatures (over 22°C) produces males, while lower temperatures (under 21°C) produce females. In other reptiles with TSD, you generally either get a pattern of females being produced at high temperatures and males at low temperatures, or females being produced at both high and low temperatures, and males produced at intermediate temperatures.
Two studies on reproduction in reptiles have made me go “wow, thats cool” this week. Read the rest of this entry »
Gene duplication is a major source of genomic novelty for evolution to work on. When genes duplicate, the extra copy of the gene is often redundant – it might degrade and become a pseudogene or take on a completely new function. Alternatively, the function of the original gene might become partitioned between the two duplicates in a process known as subfunctionalization. An excellent example of this has recently been reported in the genes that control male and female organ development in the flower, and it’s (almost) all down to a single amino acid change between the duplicate genes.
Development of male and female reproductive organs in flowers is controlled largely by a group of genes called MADS-box transcription factors. Different versions of these transcription factors (known as A, B or C function genes) are expressed in different parts of the developing flower, acting either alone or together to produce sepals, petals, stamens (male) or carpels (female)*.
Much of what we know about flower development comes from studies on two “model” plants – Arabidopsis (rockcress) and Antirrhinum (snapdragon). In these species, and in many other flowering plants, the MADs-box C-function gene that controls the production of carpels vs stamens has duplicated. In Arabidopsis, one of the copies (called AG) makes both male and female organs, but the other copy has taken on the completely new function of making seed pods shatter (and is appropriately called SHATTERPROOF). However, in Antirrhinum both copies still play a role in sex organ development: one copy (called FAR) makes only male parts, while the other copy (PLE) makes mainly female parts but also has a small role in making male parts.
Thus in Antirrhinum, the function of the original gene (making both male and female parts) has almost been split between the two duplicate copies. In a study published online in PNAS last week, researchers at the University of Leeds, led by Professor Brendan Davies, found a surprisingly simple difference in the two copies has led to their profoundly different roles. Read the rest of this entry »
In mammals, sex is determined by genes contained on sex chromosomes – males have an X and a Y chromosome, and females have two X chromosomes. In birds things are quite different, as it is the male that has two of the same type of sex chromosome. Male birds have two Z chromosomes and female birds have a Z and a W chromosome. In mammals, the Y chromosome contains a gene called SRY, which “switches on” the male sex determining pathway. So if you have the SRY gene you develop testes, and if you don’t you develop ovaries.
Until now, the identity of the master sex-determining gene in birds has been a mystery. The Z and W chromosomes of birds are not related to the X and Y chromosomes of mammals, and birds do not have an SRY gene. Research published in Nature yesterday appears to have solved this mystery, with evidence that the DMRT1 gene, located on the Z chromosome, is the bird sex determining gene. Read the rest of this entry »
Reptiles do all sorts of crazy things when it comes to sex determination. Some use good old fashioned sex chromosomes like mammals do, but in other species the temperature that the egg is incubated at determines the sex of the offspring. For instance in the tuatara, eggs that are incubated at 23 degrees are uniformly male, and eggs incubated at 18 degrees always turn out female. Some reptiles seem to use a combination of both – they have sex chromosomes, but these are overridden when the eggs are incubated at extremely high or low temperatures. Now it seems it could be even more complicated for some species, as a recent study out of Rick Shine’s lab at the University of Sydney has found that egg size is also a determining factor. In their study of the Eastern three-lined skink (Bassiana duperreyi), they found that sex chromosomes, temperature and egg size interact to determine the sex of the offspring, with larger eggs incubated at low temperatures producing females, regardless of their sex chromosomes.
Ed Yong from Not Exactly Rocket Science has a nice write-up about this paper – go check it out!
The paper citation is: Radder, Pike, Quinn and Shine (2009) Offspring Sex in a Lizard Depends on Egg Size. Current Biology in press doi:10.1016/j.cub.2009.05.027