Tuatara tuesday – sex determination in a warming world

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

Tuatara tuesday – an iconic parasite for an iconic species

As you might expect from an animal that is so evolutionarily distant from its nearest relatives, the tuatara also has some unique parasites to call its own.  One of these is the tick Amblyomma sphenodonti (sometimes also called Aponomma sphenodonti), pictured here.

Tuatara ticks Amblyomma sphenodonti

Like many ticks, A. sphenodonti are host-specific, spending all three of their life stages feeding on tuatara but dropping off into the soil in between stages.

So why should you care about tuatara ticks?  Well, these ticks are evolutionarily distinct in their own right, and are actually quite rare – far rarer than the tuatara themselves.

Tuatara tuesday – Spring fever

I haven’t had much time for writing this week, so instead I thought I’d share this photo as a reminder to my New Zealand readers that it is actually spring, even though it doesn’t feel like it!

 

Tuatara basking in the daisies on Stephens Island

 

Tuatara tuesday – how cold is too cold for a tuatara?

Tuatara like it cold.  Unusually so, for a reptile.  While reptiles in most other countries are happiest with temperatures over 25 degrees celcius, here in New Zealand our reptiles prefer much lower temperatures.  Alison Cree’s group at the University of Otago has been investigating exactly which temperatures tuatara prefer, with a view to determining whether new populations of tuatara could be established in the southern South Island.

Tuatara Tuesday – Stephens Island

This little guy lives on Stephens Island in the Marlborough Sounds, and is affectionately known as “tree tut” to the Victoria University researchers who frequent the island.  Because he lives in a tree, of course.  His tree is along the pathway between the house occupied by the DoC rangers and the house where the researchers stay, so he has plenty of passing foot traffic to keep an eye on.

Stephens Island is tuatara central, home to a staggering 30,000 – 50,000 individuals.  Given that the island is only about 150 ha in size, this means that tuatara are EVERYWHERE on the island and it is sometimes difficult to avoid treading on them.  Stephens Island has an interesting history, which may have partly contributed to the high densities of tuatara found there.  A lighthouse was constructed on the island in 1893, and three houses were also built to accommodate lighthouse keepers and their families.  During World War II a radar station was set up there, and an accommodation building known as the “Palace” was constructed.  The Palace is still there and these days serves as a lab and storage shed.

The clearing of land for the construction of the lighthouse and houses, and the introduction of cattle and sheep decimated the Stephens Island forest, and photos of the island from 50 years ago or so show barren hillsides with only a few remnant patches of bush.   However, for the last 20 years a revegetation program has been in full swing and the forest is returning.  The lighthouse was automated in 1988, and the last lighthouse keeper left the island in January 1989.  The last sheep left the island in 2005, and today the only permanent human presence on the island are the DOC rangers, who live in one of the old lighthouse keeper’s houses.

Despite the human settlement and rampant habitat destruction, the only introduced predators that made it to the island were the lighthouse keeper’s cats.  These cats decimated some of the local wildlife, including the Stephens Island wren, an unusual flightless passerine which famously went extinct virtually as soon as it was discovered.  However, the tuatara population escaped virtually unscathed and cats were eradicated in 1925 after only about 30 years on the island.  Ironically, the clearing of forest on the island may have actually increased tuatara numbers, by increasing the availability of suitable nesting sites in open areas.  The island currently appears to be above its carrying capacity, and once the forest regeneration is complete tuatara numbers may decrease somewhat.

Stephens Island, with patches of regenerating forest clearly visible

Tuatara tuesday – its not a dinosaur, OK?

From today I’ll be starting a semi-regular series of posts about my favourite reptile and #1 study organism, the tuatara.  I want to start by clearing up a misconception that I see repeated time and time again, that tuatara are “New Zealand’s living dinosaur”.

Tuatara are an entirely different lineage of reptiles from the dinosaurs.  Here’s a simplified phylogenetic tree of reptiles to illustrate:

Phylogeny of reptiles, based on that of Hugall et al. 2007 with additional information from the Tree of life (http://tolweb.org/Dinosauria/14883). Dinosaurs, including the lineage that evolved into birds, are in blue.

The closest living relatives of the dinosaurs are the crocodilians (alligators and crocodiles), and birds.  In fact, it is birds that are the “living dinosaurs”, as they evolved from the Theropod dinosaurs, the lineage that includes tyrannosaurs, Velociraptors and Archaeopteryx.  Crocodilians, dinosaurs and birds are collectively known as Archosaurs.

Tuatara are in their own Order, Rhynchocephalia, which is entirely separate from the Archosaurs.  The closest relatives of the Rhynchocephalids are the squamates (lizards and snakes), but they are not particularly close relatives at all, having diverged early in reptilian evolution, around 250 million years ago.  The tuatara is the only remaining species of Rhynchocephalid living today, but back in the time of the dinosaurs (around 65-230 million years ago), Rhynchocephalids were everywhere.  Numerous different species of fossil Rynchocephalid have been found across Europe, Africa and the Americas.   However, Rhynchocephalids appear to have died out everywhere except New Zealand around the same time the dinosaurs went extinct 65 million years ago.  Why they hung on in New Zealand is a mystery, but may have something to do with the apparent lack of competition from mammals.

Modern-day tuatara share a lot of morphological features with some of the earliest Rhynchocephalid fossils.  That, combined with the fact that they are the only living species from this lineage, has earned them the title of “living fossil”.  However, it is premature to say that tuatara are “unevolved” or “unchanged since the dinosaur era”.  For one thing, we don’t actually have any ancestral tuatara fossils  from New Zealand dating back to the dinosaur era for comparison (the earliest tuatara fossil dates to a measly 16 mya), and recent studies have shown that many aspects of their morphology and biology are derived, making them just as “evolved” as any other species (but that’s the subject for another post).

So, although tuatara are the last member of an ancient reptile lineage, and still retain some morphological characteristics of these early reptiles, they are a completely different type of reptile from dinosaurs.  Their ancestors lived alongside the dinosaurs, but so did the ancestors of modern-day turtles, crocodiles, lizards, and snakes.

References:

Hugall AF, Foster R, Lee MSY (2007) Calibration choice, rate smoothing, and the pattern of tetrapod diversification according to the long nuclear gene RAG-1. Systematic Biology 54: 543-563

Jones MEH, Tennyson AJD, Worthy JP, Evans SE, Worthy TH (2009) A sphenodontine (Rhynchocephalia) from the Miocene of New Zealand and palaeobiogeography of the tuatara (Sphenodon). Proc. R. Soc. London B 276: 1385-1390.

Tuatara: one species or two? (re-post)

The chicken or egg blog family is on holiday in Germany during August, so I probably won’t have a chance to write any new posts.  To keep you all entertained, I’ll be re-posting some of my earlier (pre-Sciblogs) articles. This post was written in July 2009.

New Zealand’s most iconic reptile, the tuatara, is currently regarded as two separate species – Sphenodon guntheri, which is found naturally only on North Brother Island in Cook Strait, and Sphenodon punctatus, which are found on other islands in Cook Strait and off the north-east coast of the North Island.  However research just published online in Conservation Genetics shows that Sphenodon guntheri is not as genetically distinctive as first thought, and suggests tuatara should be regarded as one species. 

Chemical attraction? (re-post)

The chicken or egg blog family is on holiday in Germany during August, so I probably won’t have a chance to write any new posts.  To keep you all entertained, I’ll be re-posting some of my earlier (pre-Sciblogs) articles.  This one is the very first post I wrote for this blog, from March 2009.

Research published in last month’s Chemistry and Biodiversity journal heralded the discovery of a new compound “tuataric acid”. Yes, isolated from our very own tuatara.

Stefan Schulz and his colleagues at University of Braunschweig, and collaborator Paul Weldon at the Smithsonian Institution, have analysed the constituents of the cloacal secretions in tuatara and found an unexpectedly diverse array of compounds. As tuatara have no external sexual organs, the cloaca is the “one stop shop” opening at their posterior end, with prominent skin glands on either side of the opening that secrete a greasy white substance. When the tuatara secretions were analysed, Schulz and colleagues found over 150 different types of glyceride-based molecules, including one never-before seen compound, which they dubbed “tuataric acid”.

Tuatara holds clues to human evolution

A while ago I wrote about the value of genome sequences, not just for helping us understand the biology of a particular organism, but also for enabling large-scale comparisons across species that can help spot patterns in genome evolution which wouldn’t otherwise be apparent.  A recent paper in Journal of Heredity by Craig Lowe, David Haussler and colleagues at the University of California provides an excellent example of this in action, using sequences from the tuatara genome to identify the evolutionary origin of parts of the human genome.

Lowe and colleagues were looking for functional elements (like parts of genes and their regulatory regions) in the human genome that originated from retrotransposon insertions.  Retrotransposons are mobile bits of DNA that have a tendency to make copies of themselves and insert themselves in various different places in the genome.  They contain everything needed for this copying, plus often include functional modules like exons of genes, or transcription factor binding sites.  These functional modules may be co-opted for a new function in the new site, a process known as exaptation.  Once a retrotransposon is inserted in a new location it is often inactivated, and then begins to accumulate mutations which render it unrecognisable as a retrotransposon. This makes it difficult to identify exaptation events in any given genome and hence trace the origin of many of the functional elements of that genome.  However, by comparing the genomes of many different species in different lineages it may be possible to identify ancestral versions of these elements, and so trace their evolutionary history.

Lowe and colleagues found a previously unknown retrotransposon in the small part of the tuatara genome that has been sequenced.  This retrotransposon is of a type known as a LINE – Long Interpersed Nucleotide Element - and was named EDGR-LINE  (endangered-LINE).  A search of human genome against this sequence found 18 elements that are likely to be the result of insertion of this retrotransposon into the genome at some point in evolutionary time.  Seventeen of these elements are gene regulatory regions and one is an exon of a gene called ASXL3.  ASXL3 is important for regulation of other genes during development and the additional exon co-opted from EDGR-LINE appears to help control its expression.

These 18 exaptation events likely occurred early in mammalian evolution, but the retrotransposon itself has long since been inactivated in humans so all traces of it have been lost.  The functional elements it contained are able to be identified because they are under strong purifying selection (i.e. have not accumulated many mutations), so can still be aligned with the tuatara sequence.  Its only through this comparison that it is possible to know that these 18 elements originated from the same retrotransposon.

EDGR-LINE was also found in the lizard, frog, and coelecanth, but no traces of it remain in mammals, crocodylia and birds.  EDGR-LINE appears to be more slowly evolving in tuatara than in lizards, so is closest to the mammalian ancestral version of EDGR-LINE and hence more informative for identifying elements in the human genome. In fact, 10 of the 18 elements could only be identified by comparison with tuatara and not with these other species.

Evolution of the EDGR-LINE in vertebrates. The EDGR-LINE appears to have been introduced in the common ancestor of tetrapods and lobe-finned fish, and lineages where the LINE was active are shown with green. The LINE is not noticeable in mammals, crocodylia, aves, or testudines, so it has already been inactivated at least twice in evolution.

This is not the only example of genomic information from a rare species shedding light on the evolutionary history of human genome.  The genome of the threatened desert tortoise Gopherus agassizii also harbours an ancient LINE that has enabled functional elements of the human genome to be identified.  Lowe and colleagues speculate that this may be due to the very nature of endangered species, and ran simulations to show that theoretically, mobile elements like LINEs are active for longer and evolve more slowly in small populations.   This effect comes about because of the relationship between population size and selection – selection is more efficient in large populations so is more likely to remove genetic variants which are mildly harmful (or deleterious) to the organism, and to fix mutations which are beneficial.  The smaller the population, the more likely it is that deleterious genetic variants will become fixed in that population and beneficial mutations will be removed.  Insertion of mobile elements into new places in the genome is almost always deleterious, as it messes with existing genes and their regulatory regions.  Thus small populations will be more likely to accumulate additional copies of the mobile elements, and less likely to accumulate mutations which would remove or inactivate them.  I should point out here that tuatara are not actually classified as endangered (as the paper claims), but they have had a historically low population size, with probably a severe population bottleneck during the oligocene inundation of the New Zealand land mass.  In addition, we now know that even large tuatara populations can have a small effective population size, as few individuals actually contribute to mating at any one time.

Lowe and colleagues point out that without the tuatara, we would not have been able to identify these particular functional elements in the human genome, and that we never know what additional information about human evolution we might glean from threatened species in the future.  This underscores the importance of projects like the Genome10K initiative to sequence 10,000 vertebrate genomes.  Of course I would add that we should preserve these species for their intrinsic worth not just because of what they can tell us about human evolution, but this paper does highlight the unexpected ways that genomic data from diverse species can help us understand evolution.

Lowe, C., Bejerano, G., Salama, S., & Haussler, D. (2010). Endangered Species Hold Clues to Human Evolution Journal of Heredity DOI: 10.1093/jhered/esq016

How hard can a tuatara bite?

As a geneticist, I’m only rarely let out of the lab to chase after my study animal, the tuatara.  I count these occasions as a gift, where I get to feel like a “real” biologist and learn to talk knowledgably about the ecology and habits of tuatara (which, lets face it, are generally of more interest to the lay person than their genes).  I also count myself lucky that I’ve never been bitten by a tuatara – although I have helped extract other people’s fingers from the mouths of tuatara and can confirm that it is an eye-watering experience.

We now know exactly how hard a tuatara can bite, thanks to a recent study published in the Journal of the Royal Society of New Zealand.  Marc Jones (University College London) and Kristopher Lappin (California State Polytechnic University) have measured bite force in adult tuatara and found that a male tuatara could produce a bite force of up to 238 Newtons.  Jones and Lappin measured bite force using a custom-designed isometric force transducer.  They report that the tuatara needed little encouragement to bite onto the leather-covered bite plates, and that “once biting commenced the tuatara would maintain its grip with considerable reluctance to release”.  Something that will come as no surprise to those who have been on the receiving end of a tuatara bite!