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Tuatara tuesday – sex determination in a warming world Hilary Miller Nov 09

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Seeing as reptile reproduction seems to be a bit of a hot topic right now, I thought it was time to talk about sex determination in tuatara.

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.

Tuatara hatchling

Dr Nicola Nelson at Victoria University has experimented with switching tuatara eggs between male and female-producing temperatures in an effort to determine which part of the incubation period temperature is critical for sex determination.  She found that sex is set early on - by the time the incubation period is about one third of the way though.  However, incubating eggs in captivity at constant temperatures only tells part of the story, as of course temperatures are not constant in the wild, where eggs are laid in shallow burrows in the soil.  Nelson and colleagues have also collected temperature data from natural nests and found that warmer nests produce males and cooler nests produce females, but what isn’t known is how long eggs have to remain above or below the critical temperature in order to produce males vs females.  Its possible that the critical period for sex determination is actually quite short – for example an egg may actually only need to spend a few days above 22°C in order to turn out male.

A partially buried tuatara nest on Stephens Island

Having more males produced at warmer temperatures could be bad news for tuatara in the light of global warming.  Some tuatara populations, like the small, genetically distinct population on North Brother Island, already have more males than females.  A recent study by Nicola Mitchell of the University of Western Australia predicted that, under current “worst-case” global warming scenarios, populations like North Brother Island will produce all-male clutches by the mid 2080s.

Of course, tuatara have survived changes in climate in the past, but this time around the climate is changing faster than ever before – perhaps too fast for a species like tuatara with its long generation times and low levels of genetic variation to be able to evolve to compensate.   Tuatara may be able to adapt behaviourally to the higher temperatures by nesting earlier, digging deeper nests, or choosing cooler nest sites.  However, on many islands the choice of nest sites is limited, and as tuatara are now confined to offshore islands or ringed in by predator-proof fences on the mainland, they will be unable to simply move south to seek cooler temperatures. It seems likely that tuatara will need our help if they are to survive the threat of global warming.

Further reading:

Mitchell NJ, Kearney MR, Nelson NJ, Porter WP (2008) Predicting the fate of a living fossil: how will global warming affect sex determination and hatching phenology in tuatara? Proceedings of the Royal Society B-Biological Sciences 275: 2185-2193

Huey RB,Janzen FJ (2008) Climate warming and environmental sex determination in tuatara: the Last of the Sphenodontians? Proceedings of the Royal Society B: Biological Sciences 275: 2181-2183

Mitchell NJ, Nelson NJ, Cree A, Pledger S, Keall SN, Daugherty CH (2006) Support for a rare pattern of temperature-dependent sex determination in archaic reptiles: evidence from two species of tuatara (Sphenodon). Frontiers in Zoology 3: 9

 

The weird ways of reptile reproduction Hilary Miller Nov 04

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Two studies on reproduction in reptiles have made me go “wow, thats cool” this week.

Firstly, the report of a boa constrictor giving birth to two litters of offspring without the need for a father.  This sort of “virgin birth” is called parthenogenesis, and is not that uncommon in itself, having been previously observed in a few other reptile and fish species, and numerous invertebrates.  What separates this latest report from the others is the unusual complement of sex chromosomes observed in the offspring.

When vertebrates reproduce by parthenogenesis, the offspring are usually “half clones” of the mother.  Chromosomes come in pairs, and normally you get one half of the pair from your father and the other half from your mother.  But in parthenogenesis, both halves come from the mother – that is, two copies of one half of the mother’s chromosomes are inherited.   This means that when it comes to the sex chromosomes, the offspring end up with two of the copies of the same chromosome.  In all other recorded instances of pathenogenesis in vertebrates, species like snakes with the ZW sex chromosome system (where males have ZZ and females have ZW chromosomes) produce only male offspring, and species with XY chromosomes (males XY and females XX)  produce only female  offspring.   In other words, only the sex where the two sex chromosomes are the same is produced and the opposite scenario (WW females or YY males) was thought to result in non-viable offspring.  Until this boa constrictor came along, that is -  yep, her litters are made up entirely of WW female snakes, a finding which “up-ends decades of scientific theory on reptile reproduction”.

This study was published online this week in Biology Letters, and the BBC news has more on the story here

The second study, published online in Nature this week, is an great example of just how malleable sex determining systems can be.  For many reptiles, sex is determined not by sex chromosomes, but by what temperature the egg is incubated at.  In tuatara for example, incubating eggs at high temperatures produces males, while low temperatures produce females.  You might think that chromosomal and temperature-dependent sex determination are two fundamentally different, mutually exclusive ways of determining sex.  However, in some species the line between these two systems is blurred, suggesting that switching between systems is easier than you would think.  This study found that the Tasmanian snow skink (Niveoscincus ocellatus) has both types of sex determination, and all it has taken to switch between the two is a shift in climate. 

The snow skink lives in both the warm lowlands and cool highlands of Tasmania.  The study found that in the highlands, the skink uses chromosomal sex determination, producing an equal ratio of male and female offspring regardless of the temperature.  However, in the lowlands its sex determination is temperature-dependent - cool cloudy days produce males, and warm, sunny days produce females.  The researchers speculate that the divergence in sex determination mechanisms was caused by temperature differences, enabling the lizards to maximise their reproductive output in the differing climates. 

The paper is here, and ABC news has a report on this here.

A simple change determines male vs female organ development in flowers Hilary Miller Oct 30

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ResearchBlogging.org 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.

Davies and colleagues created chimeric versions of PLE and FAR, swapping domains between the proteins to determine exactly what parts of the different proteins are responsible for their differing function.  They narrowed down the difference between the two genes to a single amino acid that is present in FAR but not in PLE. When this amino acid was removed from FAR, the gene switched to making both female and male parts.  FAR and PLE are estimated to have duplicated around 120 million years ago, and the researchers estimate that the mutation responsible for inserting the extra amino acid into FAR happened around 20 million years after the duplication.

Duplicated genes often take on new functions because changes in their regulatory regions change how and where they are expressed.  Thus, finding an example such as this one, where a simple change in the protein coding sequence causes a profound change in function is somewhat unusual.  However, these proteins don’t act in isolation – they are just one part of a network of genes that must work together to control sex organ development.  Davies and colleagues found that the single amino acid change alters the ability of the protein to interact with other proteins in this network.

The additional amino acid in FAR is found in the part of the protein that interacts with other types of MADs-box proteins called SEP proteins.  C-function genes without the additional amino acid (like PLE) can interact with 3 different SEP proteins (SEP1, SEP2 and SEP3), but proteins with the additional amino acid (like FAR) can only interact with SEP3.  The SEP3 gene is not expressed in the first whorl of the flower, where female parts are produced, so FAR doesn’t have anything to interact with in this whorl and therefore doesn’t produce female parts.

Davies describes this as “an excellent example of how a chance imperfection sparks evolutionary change”.  It is also a nice example of subfunctionalization in action, where a simple amino acid change provides a means of separating the functions of the duplicate copies by causing a change in how the protein operates in a larger regulatory network.

Reference: Airoldi CA, Bergonzi S, & Davies B (2010). Single amino acid change alters the ability to specify male or female organ identity. Proceedings of the National Academy of Sciences of the United States of America PMID: 20956314

*For more on the ABC model of plant development, see here or here.

Bird sex gene identified Hilary Miller Sep 11

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ResearchBlogging.org 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.

baby chickensDMRT1 has long been suggested as a good candidate for the ’master switch’ gene in birds, but until now this has been difficult to prove due to the technical difficulty in knocking out expression of genes in large, yolky bird eggs.  Craig Smith, Andrew Sinclair and colleagues from the University of Melbourne have now succeeded in silencing expression of DMRT1 in chicken embryos using a method called RNA interference.  They found that in genetically male chicken embryos where levels of the DMRT1 protein were reduced by RNAi, the gonads developed into ovaries rather than testes, suggesting that DMRT1 is required for testes development.  These embryos also showed reduced expression of a male marker gene SOX9, but levels of a female marker gene aromatase (which is not present in normal male embryos) increased.  Reducing DMRT1 expression had no effect on genetically female embryos - their gonads developed into ovaries as usual.

These results suggest that DMRT1 alone can determine whether an embryo becomes male or female.  However, unlike in mammals, where sex is determined by presence or absence of SRY, DMRT1 appears to act in a dosage-dependent manner.  Male birds have two copies of the gene (one on each Z chromosome) so produce twice as much DMRT1 protein as female birds.  The results of Smith and colleagues suggest that DMRT1 levels need to reach a certain threshold before the male developmental pathway is switched on, and this level can only be reached by birds with two Z chromosomes.

Unfortunately, Smith and Colleagues were unable to conduct the converse experiment to test whether you can turn genetically female embryos into males by overexpressing DMRT1.  This is because their experiments resulted in overexpression of DMRT1 in all tissues, not just the gonad, which is lethal to the developing embryo.  Future refinements of their method should enable them to direct expression to the gonad only, mimicking what would happen in a genetically male embryo and confirming the role of DMRT1 as the master sex switch.

This study fills a large hole in our understanding of sex determination in vertebrates.  DMRT1 appears to be involved in sex determination across a variety of vertebrates, suggesting it has an ancient role.   This is backed up by the fact that early mammalian lineages and some reptiles have sex chromosomes related to the ZW system of birds, not the XY system of later mammals, suggesting that SRY is only a recent arrival on the sex determination scene.

Smith, C., Roeszler, K., Ohnesorg, T., Cummins, D., Farlie, P., Doran, T., & Sinclair, A. (2009). The avian Z-linked gene DMRT1 is required for male sex determination in the chicken Nature, 461 (7261), 267-271 DOI: 10.1038/nature08298

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