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Entries in evolution (11)

Saturday
Jun102017

Journal club: the origin of menopause

Menopause is, at face value, evolutionarily perplexing. Why select for the inability to breed, considering natural selection works only on the success of having offspring? Despite several claims, menopause is actually exceptionally rare - only humans and resident orcas (killer whales) have a systematic post-reproductive menopause state. It is rather hard to dissect the why in humans, but this study gives a good explanation of the demographic situation needed to evolve menopause. In short:

* Demographic situations where the offspring of mothers and daughters are in competition, plus where the offspring of daughters have an advantage (if the offspring of mothers have an advantage, breeding suppression in the young is observed)

* Social groups where the "grandmother" effect can aid the survival of the group (food sharing)

The implication is that these were key factors during human evolution, which would have molded our behaviour as well as our biology to maximise fitness.

Friday
Apr072017

Journal club: Did giant viruses shrink, or did small viruses grow?

The smallest viruses can be just a few nanometres wide and contain two genes. Viroids can get even smaller. Recently, however, giant viruses have been found, 1000-fold larger and with more genetic content than some bacteria.

This raises a fascinating question as to the origin of viruses. The first model is that viruses were spawned out from some proto-bacteria-like cells. A bit of bacterial DNA, like a plasmid, that was able to survive cell-free and move from cell to cell. In this view, viruses don't really belong on the tree of life, and giant viruses are just abnormally large viruses that have captured more and more DNA from hosts.

Giant viruses raise a second model. What if the tree of life had an original fourth strand (bacteria, eukaryotes, Archaea and proto-viruses). The first three strands are still with us today, while the proto-viruses have devolved from a free-living cell to a dependent virus. Under this alternative view, giant viruses are ancient relics closest to the original proto-viruses, with the smaller viruses having gone further down the evolutionary pathway of becoming highly efficient replicators.

A paper published in Science this week argues for the first model, with evidence that some key machinery in giant viruses is directly stolen from other lineages of the tree of life. But the argument is not closed, and as more and more giant viruses are found, I look forward to seeing which direction the evidence moves.

Saturday
Jan142017

Journal club: A new hint to the evolution of the nucleus?

One of the fundamental divisons of life is between prokaryotes and eukaryotes. Prokaryotes, including both Archaea and bacteria, have a very simple cell, with no major subcellular compartments. Eukaryotes, essentially all over living things (plants, animals, fungi, amoeba, etc), have much more complex cells, with a nucleus and dedicated organelles (mitochondria, chloroplasts, etc).

Exactly how eukaryotes evolves from prokaryotes is one of the biggest questions in evolution. It is clear that some organelles, such as mitochondria and chloroplasts, evolved by endosymbosis. Essentially, one bacteria swallowed another, which survived inside and took on specialised functions. The nucleus has been a bigger mystery, with no clear preceeding structure in prokaryotes to build upon. Until now. In the lastest issue of Science, Chaikeeratisak et al discover a nucleus-like structure in bacteria. Interestingly, the structure is not part of the bacteria's normal lifecycle, instead it is assembled by a virus, bacteriophage 201φ2-1, as a structure for DNA replication and transcription, after which the RNA is sent to the cytoplasm of the bacteria for translation, exactly as occurs in eukaryotes.

There are many fascinating angles to this paper. Why does bacteriophage 201φ2-1 assemble this structure? Is it to protect the phage genome from bacteria attack by restriction enzymes and the like? Most importantly, could similar viruses have been the origin of the eukaryotic nucleus? One can easily imagine an ancestral prokaryote becoming permanently infected by such a virus, and rapidly evolving useful functions for a nucleus of its own. 

 

Read the paper: Vorrapon Chaikeeratisak, Katrina Nguyen, Kanika Khanna, Axel F. Brilot, Marcella L. Erb, Joanna K. C. Coker, Anastasia Vavilina, Gerald L. Newton, Robert Buschauer, Kit Pogliano, Elizabeth Villa, David A. Agard and Joe Pogliano. 'Assembly of a nucleus-like structure during viral replication in bacteria'. Science  13 Jan 2017: Vol. 355, Issue 6321, pp. 194-197

Friday
Apr032015

Evoluation in action: baby sting-ray edition

Not just an excuse for a cute picture - look at the hind limbs that develop in the baby sting-rays - completely non-functional in the adult, but you can see that they are part of the developmental process. Evolution is messy, and always works from what is pre-existing, rather than what would be ideal.

Wednesday
Jun272012

If birds are baby dinosaurs and humans are baby apes...

A recent publication in Nature suggests that in many ways, birds are baby dinosaurs. The finding is less unusual than it might seem, afterall it is well established that humans have many traits of baby apes and dogs are in some ways baby wolves. The process is known as paedomorphosis or neoteny - the retention of juvenile traits in the adult form. This can take the form of enlarged eyes (birds), larger brains (humans) or retention of juvenile behaviour (dogs).

The reason why paedomorphosis works is that the basic body plan has much deeper evolutionary roots than the species-specific add-ons. Think of it this way, all mammals pretty much have the genetic program to make a nose, but only the elephant has evolved an additional genetic plan to turn that nose into a trunk. Deep in the genetic code of the elephant there is still the "standard nose" code (and indeed, the foetus has a relatively normal nose), it just has added lines of code that upgrade the standard nose into a trunk. This means that in theory, the elephant could evolve away from the trunk by just ditching the upgrade code, letting it default into the standard nose code. This is true for most of development - new code is never optimally created for the organ, rather it is always adding a bit of extra code to change the outcome. For a software engineer it would be the hight of laziness, creating bloated useless code, with every problem solved by kludge

Despite being inefficient and inelegant, the system of "generic code" plus "species specific" is very useful for evolution. This is because species evolve to be adapted to a specific environment. The flamingo beak is fantastic for a filter-feeder, but it has lost the generic functions that a sparrow could use its beak for.

Imagine an island with brine lakes that is populated only by flamingoes. If those brine lakes dried up, the flamingoes would go extinct. But what if new niches opened up? The ordinary "forward" process of evolving a generalist beak is quite slow, because you need to generate new code, but the "backwards" process of paedomorphosis could be quite fast, because it is just the process of deleting the species-specific code, defaulting back to the generic beak (as in anything else, destruction is faster and easier than generation). It is not difficult to imagine a relatively small set of genetic deletions that would mean the adult flamingo retained the juvenile generic beak, and then these "de-evolved" generalist birds could take advantage of the new habitat, and indeed start to evolve specific changes to specialise towards that new habitat.

As a general rule, following a large change in the environment, the generalised (juvenile) body plan is probably going to be more successful than the specialised (adult) body plan. Paedomorphosis in effect provides a default option to revert to in case of catastrophic change, allowing a species to shed its specialised features and start again. One possibility that interests me is that an open niche may drive paedomorphosis by selecting for rapid population growth. Consider the drying up of Africa that occured 5 million years ago. All of the apes that were specialised to live in rainforest would have seen dramatic contraction of their habitat, leaving just a few thousand gorillas left today. But the drying also created a new niche, the savanna, which could be exploited by any ape that was able to adapt. Paedomorphosis probably played a role in human evolution, by shedding the arboreal features required to swing in trees, allowing the pre-humans to venture onto the savana. Now consider the first pre-humans that were suitable for the savana - they has a continent to spread across, with the only limitation being the reproduction rate. We already know that a truly open niche creates an evolutionary pressure to fill it - such as the natural selection of cane toads in Australia with longer legs simply because they can move faster into virgin territory. What if this put selection on humans to reproduce at a younger age? Any variants that became fertile younger (and thus, while still carrying juvenile features) would outcompete the others, creating a population shift. In effect, there would be selection for paedomorphosis simply to increase the reproduction rate, with the retention of other juvenile traits (such as a larger brain) being a side-effect. 

If this model it correct, it would mean that open niches would drive paedomorphosis via two mechanisms - by selecting for the retention of juvenile traits to give a more generalist body plan, and by selecting for sexual maturity at a younger age to give more rapid reproduction. This dual selection force would drive much more rapid evolution, and may be responsible for some of the most remarkable evolutionary shifts, including the evolution of humans. 

Friday
Jan202012

Generation of a family-specific virus through repeated human passage

Generation of a family-specific virus through repeated human passage

Hayden A M Liston1, Lydia E Makaroff1 and Adrian Liston 1,2*
1 Sleepytown University, Brussels 1060, Belgium
2 VIB, Leuven 3000, Belgium
*send correspondance to adrian.liston@gmail.com

Nature Junior 8(2) 103-7 

Background. Effective control over viral infection relies on the host carrying appropriate HLA alleles for viral antigen presentation. The explosive expansion of viruses like small-pox into previously isolated human populations demonstrates the potential for certain viral strains to have a disproportionate effect on particular racial groups. As yet, however, a virus with pathogenic potential restricted to the family level has not been identified. Objective. To generate a family-specific virus in an experimental setting, in order to test the feasibility of this occurrence in nature. Methods. A common cold virus was repeatedly passaged between two related individuals for six months. Mechanisms of transmission included frequent kisses, the placement of hands and feet into the mouth and in one instance direct vomiting into the mouth. Results. A single viral strain was propagated with the capacity to chronically infect both members of this family, while having seemingly non-pathological consequences upon exposure to unrelated individuals. The pathogenic loci are predicted to be a dominant HLA carried by both family members, as the experimental inoculation of a third individual, related to one family member but not the other, did not result in pathology. Conclusions. Generation of a family-specific virus is feasible through repeated experimental transfer between family members. A natural situation analogous to the experimental set-up used here would be the transmission that can occur between parents and young children with low levels of personal hygiene. The dominant activity of the HLA cluster in this infection suggests the generation of a regulatory T cell population which inhibits effective immunity against the family-specific virus.

Key Words: virus, horizontal transfer, HLA, human genetics, regulatory T cell.

Saturday
Jan162010

Sex determination

If yeast sex is simple, how complicated is sex in multicellular organisms? Actually, the act of sexual reproduction in plants and animals (including humans) is essentially identical to that of yeast, fungi, plants and animals, including humans. Multicellular organisms have two different copies of the genome in every cell. Like yeast, to undergo sex we need meiosis to occur. Specialised sexual cells duplicate the two genomes, cut and paste them into four unique genomes, and then divide into four daughter cells. These cells are either large cells containing a single genome and lots of energy (the female egg) or small cells containing nothing more than just a single genome (the male sperm). When they combine the new cell has two genomes, one from the female parent and one from the male parent. Importantly, just like yeast, the offspring that results has a unique genetic composition. The two genomes that the offspring possesses are each novel, created by the combination of the two unique genomes in each parent.

While sex for multicellular organisms is identical to yeast at the cellular level, at the sex determination level things get very complicated. There are many different ways for determining whether an individual is male or female. As a measure of the broad diversity of ways in which species have solved the sex determination problem we can look at a few different examples: clownfish, crocodiles, humans, whiptail lizards and komodo dragons. And this is not including some of the really complicated systems that exist, such as in earthworms, bees and platypi.

Clownfish and crocodiles

Clownfish and crocodiles both have non-genetic sex-determination systems. Males and females have the same genetic make-up and every genome has potential to encode either a male or female individual. The physical manifestations of sex occur due to environmental influences on which set of genetic controls to activate. In clownfish the important environmental influence is the social interaction of other clownfish. All clownfish start out as males. When the sole female in the group dies, the largest male undergoes a rapid sex change and becomes a female. Interestingly, this sex change is reversible – a female moved into a new group where she is no longer the largest will revert back to a male. This plasticity ensures that there is always a breeding female in every group, and that the female comes from the most successful individual in the group.

Like clownfish, crocodiles have no genetic difference between males and females. Unlike clownfish, however, there is no sex plasticity. A male hatches as a male and stays a male for life, a female hatches as a female and stays a female for life. The important environmental influence in this case is the temperature of the egg. If the temperature of the egg is between 31.7°C and 34.5°C the embryo is set as a male, if the temperature of the egg is outside this range the embryo is set as a female. There are several important restraints that this sex determination system has had on crocodile evolution. Firstly, the crocodilian mother has become very active in nest maintenance, as a temperate far from the threshold will result in hatchlings of a single sex. Secondly, this sex determination system has forced crocodiles to maintain a link to land. Other aquatic species, such as dolphins and sea snakes, have been able to become entirely aquatic by giving live birth in the water. These species all have genetic sex determination systems (below). By contrast, crocodiles and turtles need to return to land to lay eggs because a temperature-dependent sex determination system is incompatible with live birth – internal body temperatures are too stable to give the diversity in temperatures required.

Humans and whiptail lizards

Humans and whiptail lizards both use the XX/XY sex determination system. In this system, sex is determined by the combination of sex chromosomes inherited from the parents. XX results in females and XY results in males. As females can only pass on an X chromosome while males can pass on either an X (50% chance) or Y (50% chance) chromosome, this system results in roughly equal numbers of females and males being born. It is very important to note that differences between the sexes are largely not due to genetic differences. Both males and females have the X chromosome, and while females have two copies one of these copies is “inactivated”, making them equivalent to males. The only substantial genetic difference between males and females is the presence of the Y chromosome in males. This Y chromosome is tiny (only 2% of the human genome) and is mostly made of up junk. The only essential gene on the Y chromosome is the SRY gene.

All human embryos, whether XX or XY, spend the first six weeks as females. At this point embryos with the XY genome express the SRY gene in the genital tissue, starting the development of testes. The testes then express testosterone and the embryo detects this testosterone production through the androgen receptor. The effect of this production is a complete remodelling of the genitalia from female into male between 7 and 12 weeks gestation. Many different genes are used to initiate the “male” program instead of the “female” program, but only the SRY gene is on the Y chromosome. In other words, females have all the genes required to develop the physical attributes of a male, and males have all the genes required to develop the physical attributes of a female, and only a single gene decides which program is used. When thinking about physical differences between males and females it is not helpful to think about genetic variation, such as exists between different populations of humans. Instead the best comparison is to think about your heart and your liver. Both cells have the same genome, the same genetic code, but the two cells have initiated different programs from the same code so that the cells can perform different functions.

Most whiptail lizards use the same XX/XY system as humans, with XX lizards being female and XY lizards being male. However 15 species of whiptail lizards have reverted to an asexual system of reproduction. These species consist only of XX females. The females still undergo sexual meiosis to create an egg with a single X chromosome, however in the absence of sperm these eggs spontaneously duplicate their genome to become XX females, in a sexual system called parthenogenesis. It is unclear as to why these whiptail lizards have evolved to abandon the advantages to sexual reproduction, however a clue may be the environment they live in – the dry deserts of North America. It is likely that with the low population densities of lizards living in a desert finding a mate becomes very difficult. By breeding through parthenogenesis females can still reproduce even if they fail to find another lizard, and furthermore every individual offspring is capable of bearing young, allowing more efficient use of resources during dry times, and faster population growth during wet ones.

Komodo dragons

The ZW/ZZ sex determination system used by Komodo dragons is essentially the opposite of the XX/XY system. Here, ZW results in a female while ZZ results in a male. As with the Y chromosome, the W chromosome is a minor chromosome with few functions beyond sex determination. When breeding, a female Komodo dragon can pass on either a Z or W chromosome while a male Komodo dragon can only pass on a Z chromosome. This results in 50%:50% females to males. Interestingly, the Komodo dragon has also developed parthenogenesis, like the whiptail lizard. A female Komodo dragon kept alone will have spontaneous genome duplication of an egg. The outcome, however, is the opposite of that occurring in the whiptail lizard. The whiptail lizard female, using the XY sex determination system, can only pass on an X chromosome, so duplication results in an XX female. The female Komodo dragon, however, uses the ZW sex determination system, so the egg could either have a Z chromosome and duplication to be a ZZ male, or it could have a W chromosome and duplication to become an unviable WW embryo. In practise, therefore, this means that female Komodo dragons that revert to parthenogenesis will always generate ZZ males. This means that a lone female washed up on a new island will generate male offspring by parthenogenesis, allowing later sexual reproduction. What is the advantage of this system of parthenogenesis? There are two likely possibilities. The first is that it avoids the spiral into an inbred population that occurs in whiptail lizards, with only a single necessary parthenogenic generation interrupting sexual reproduction. This may be a more appropriate adaption to the “rich uninhabited island” scenario, with the XX parthenogenic strategy more suitable for the “low population desert” context. Alternatively, and equally plausible, the ZW parthogenesis strategy is less efficient than XX parthenogenesis in both contexts (or vice versa). Since evolution always works from the current genetic situation in incremental steps, non-ideal compromises are common.

Tuesday
Jan122010

The role of sex in evolution

Sex is a powerful force for evolution. On the face of it, sex seems like an absurdly complicated way to reproduce. Prokaryotic organisms, bacteria and archea, have a much faster a simpler system, where the cell simply duplicates its DNA and splits in half into two identical daughter cells. The entire process, called mitosis, only takes 20 minutes. This means that under ideal circumstances a single bacterium can divide to produce 8 offspring in the first hour. In the second hour that single precursor cell could form 64 offspring, after 6 hours a single cell could form over 200,000 daughter cells. This asexual reproduction is so efficient that it only operates at capacity for very short durations, as exponential growth of a single cell could use up the resources of an entire planet within days. Typically a bacterium ticks over slowly by scavenging what resources are available, only to explode into exponential asexual growth when new resources become available and a race to exploit them occurs.

Compare this to the elaborate, time-consuming and often bizarre process of eukaryotic sex, which multicellular organisms from plants to fungi to animals use to reproduce. Sex (and the accompanying mate selection) is one of the most difficult and dangerous parts of an individual’s life, and even passionate advocates of the activity find it difficult to explain. Yet through an evolutionary lens, sex provides very concrete advantages. The best illustration of the advantages of sex come from yeast mating, as these simple organisms are capable of both asexual and sexual reproduction.

Simple sex

Yeast can be thought of as being halfway between simple bacteria and complex multicellular organisms like humans. In terms of lifestyle and behaviour, yeast operate like bacteria – single celled organisms capable of an independent existence through the use of resources in their direct environment. Inside the cell, however, yeast are clearly eukaryotic organisms, with the same basic machinery for cell division, metabolism and survival as plants and animals. It is therefore convenient to think of yeast as essentially human-like cells, trapped in an early bacterial-like lifestyle. This is an oversimplification of course: bacteria, yeast and humans are all highly evolved organisms and none have remained static in evolutionary time, but it is a useful oversimplification.

So how do yeast reproduce? Asexually, like the bacteria they share a lifestyle with? Or sexually, like the multicellular organisms they are genetically closest to? The answer is both. When yeast are in a rich nutrient environment they reproduce asexually like bacteria. A single cell undergoes mitosis, duplicating its DNA and then splitting into two daughter cells, each identical to the parental cell. This gives the yeast all the advantages of bacterial reproduction – very simple rapid reproduction to win the race for abundant resources. The parental cell was successful in the environment, so the identical daughter cells should be equally successful and proliferate likewise.

However as noted above, exponential growth can never continue unabated, sooner rather than later resources become limiting or some other factor stresses the survival of the yeast. At this point yeast have a trick available that bacteria do not – sex. Instead of undergoing dormancy, the yeast mate.

In the best understood system, that of Saccharomyces cerevisiae, there are two sexes of yeast, a and a, controlled by a single gene. Mating is very simple, the a cells release a chemical called ‘a factor’ and produce a receptor that causes them to migrate towards the chemical ‘a factor’. By contrast, the a cells release a chemical called ‘a factor’ and produce a receptor that causes them to migrate towards the chemical ‘a factor’. The two yeast cells, one a and one a, attract each other and fuse into a single cell. This cell now has two different copies of the yeast genome, one from each parent.

The a-a fused yeast cell can now undergo a complicated cellular division process called meiosis. Unlike mitosis, where the cell duplicates its genome and divides in two, meiosis involves duplicating the genome and dividing in four. This is possible because the a-a fused yeast cell has two copies of the genome to start with, so duplication gives four copies, one for each of the four daughter cells that result.

The important difference between mitosis and meiosis is the splicing of two different genomes to form unique combinations. Mitosis just duplicates the existing genome. Meiosis starts with two different genomes, and during the duplication processes these genomes are jumbled up together, creating new combinations of old characteristics. This means that all four daughter cells at the end are unique and different from the original parental cells.

The advantage conferred by sex is very straight forward – the parental cells were not dealing well with the environment they were in, since yeast mating occurs only under stress. Therefore why reproduce more cells that cannot cope with the environment? Instead the yeast takes a life-or-death gamble that a combination of genetic information from another cell will produce offspring better able to deal with the environment. In a simple scenario there would be two yeast strains, one able to deal with acidity and one able to digest complex carbohydrates. A change in environment to a high acidity environment where the only resources available are complex carbohydrates will stress both parental strains. However, by sex there is a chance that one of the daughter cells will inherit the acid resistance of one parent and the ability to digest complex carbohydrates from the other parent. Other daughter cells will not be so lucky and will die, but that one daughter cell with the chance combination of two necessary characteristics will be able to divide asexually and rapidly reap the rewards of a new resource.

In one final complication, yeast can change sex. A single gene makes yeast either a or a, so after mating and meiosis the four daughter cells include two a cells and two a cells. If a single a cell is successful in the new environment, asexual reproduction creates exact copies, so all progeny will be a cells. This would create an obvious problem if a new environmental stress requires another round of mating, so yeast carry spare “silent” copies of a and a genes and use these backup copies to flip from one sex to another, to make sure a population is always a mixture of a and a yeast.

Sunday
Dec062009

The inefficient consequences of evolution

Vertebrates are unique in developing an immune system capable of anticipating pathogens that are yet to evolve. Birds and mammals have taken this "adaptive" immune system to the pinnacle, with T cells and B cells using a randomised form of genomic engineering. The advantage of a system based on randomised generation is striking - by making every T cell and B cell unique it becomes exceptionally difficult for pathogens to "out-evolve" their hosts. Regardless of how a pathogen will change, pre-existing T cells and B cells will be capable of recognising the new modified pathogen. The importance of the adaptive immune system to humans is evident in the fatal consequences of its absence, such as patients with end-stage AIDS or primary immunodeficiencies caused by genetic mutations. These benefits greatly outweigh the cost of the adaptive immune system in resources used and the threat of autoimmune disease.

But does the adaptive immune system make vertebrates more healthy? There is no obvious evidence that it does. In a key essay on the topic, Hedrick argues that vertebrates do not appear to have a lower pathogen-induced mortality rate than invertebrates. Instead, he argues that the development of the adaptive immune system provided only a short-term benefit, with pathogens rapidly being specialised to vertebrate hosts. The result is an immunological arms race, with each side incrementally ratcheting up the armaments. Vertebrates are essentially impervious to non-specialised pathogens unless rendered immunodeficient, but the additional mortality from specialised pathogens is probably equivalent to the invertebrate state.

This still-controversial hypothesis high-lights an important aspect of evolution by natural selection. It has highly inefficient consequences. Natural selection takes place at the level of the individual and evolution takes place at the level of the species. Most importantly, natural selection only occurs in the present. An individual who has an advantage for even a single generation will be over-represented in the next generation. A species that has an advantage for a single generation will be able to exploit more resources for reproduction. The long-term consequences - that each species will waste more resources in an ever more expensive battle - is irrelevant.

The evolutionary arms-race between host and pathogen is one incredibly important example. A more illustrative example of the patent futility of this arms-race comes from Sir David Attenborough, one of the leading science communicators of all time. In Life in the Undergrowth, he films two species of harvest ants living in the desert. Each population needs to collect seeds to survive, however the number of seeds produced in the desert is so low that there is fierce inter-species competition. One species of ant is diurnal, the other nocturnal, and each is capable of collecting the entire daily seed dispersal. In order to survive, every second night the nocturnal ants spend an evening carrying rocks to cover the entry hole of the diurnal ants. The diurnal ants can't collect seeds the next day as they need to spend a day clearing the rocks from the entrance. This gives the nocturnal ants a night to harvest the uncollected seeds. The following day the diurnal ants are able to collect every seed and that night the nocturnal ants spend carrying rocks. Two species end up literally carrying rocks backwards and forwards every second day.

The elegance of evolution is the beauty of such specialised behaviour, but the consequences are gross inefficiency in resource use. If each species simply spent alternative cycles conserving resources both species could survive with a higher population density than currently exists. But neither species can be the first to stop the wasteful use of resources, as that would give a fatal advantage to the other, and so they are trapped together in a cycle of carrying stones. The battles of night ants vs day ants and of hosts vs pathogens illustrate the bizarre, elaborate and ofttimes perverse consequences of evolution by natural selection

Monday
Sep212009

The evolution of sex chromosomes

An interesting study in this week's edition of Nature by Organ and colleagues looks at the evolution of sex chromosomes. While humans use the XY system for determining sex (XX for females, XY for males), this is by no means the only system for determining sex. Most reptiles, for example, determine sex by the temperature at which the young develops. For example crocodiles develop as males if the eggs are between 31.7°C and 34.5°C, and females if the eggs are above or below this temperature.

A chromosome-based method for determining sex has arisen not just once, but several times. Mammals use the XY system, but birds use the ZW system (where ZZ is male and ZW is female). These systems create problems, such as the dosage compensation question (how to stop excess / insufficient production of genes on the X or Z chromosomes in the gender with two copies / one copy), however they have a major advantage. This advantage is most evident in mammals - mammals are endothermic, meaning that we keep a constant body temperature. We also bear live young. Obviously, this combination of characteristics would be fatal to a species with temperature-dependent sex determination - all offspring would be of one sex.

In this paper the Pagel laboratory has used an evolutionary analysis to consider the relationship between bearing live offspring and having a chromosome-dependent sex determination system. There are multiple examples of animals with chromosome-dependent sex systems that lay eggs (all birds) and even examples of animals with temperature-dependent sex systems that bear live offspring (some lizards). However in one group of animals the relationship was very strong - amniotes that have fully returned to the sea (sea snakes, sirenians and cetaceans) are all live-bearing and have chromosome-dependent sex systems. An evolutionary analysis predicts that other extinct lineages of sea reptiles, mosasaurs, sauropterygians and ichthyosaurs, also developed chromosome-dependent sex systems before evolving life birth and spreading out over the ocean.

Like mammals with endothermic body temperatures, the constant temperatures of the ocean would have spelt doom to any species that evolved life oceanic birth before evolving a chromosome-based sex system. This is probably the reason why otherwise entirely aquatic species that use temperature-based sex determination systems (such as crocodiles and sea turtles) remain bound to laboriously climb out of the water to lay their eggs.