The taming of the rabbit
The genetic changes leading to domestication of the many plant and animal species we use for food and keep as pets has become a popular research topic. A consistent problem in such studies is identifying the original wild population from which domesticated species evolved, or even determining if these populations still exist.
Studies of dog domestication, for example, compare domestic breeds to wild wolves. Given the considerable reduction in wolf populations over the last thousand years however, it seems unlikely these populations are the same as those from which dogs evolved from.
Leif Andersson, of Uppsala University, presented evidence that rabbits provide a unique opportunity to avoid this problem. For most tame animals, domestication began back in pre-history. For rabbits however, we know with some exactness the date when their domestication began: around 600AD in southern France (allegedly because the Catholic Church deemed rabbits to be fish rather than meat, meaning they could be eaten during Lent). Many wild rabbit populations still exist in these regions, meaning we can feel confident in comparing domesticated rabbits with their actual wild ancestors.
This is what Andersson and his team did, sampling a considerable range of wild rabbit populations in Iberia and France and comparing their allele frequencies to six domestic rabbit breeds. They found strong enrichment of genes involved in neuronal regions and brain development; findings consistent with a change in the flight response of wild and domestic rabbits.
This is perhaps not surprising; it is often noted that while the wild relatives of most domestic species are not prone to panic, but wild rabbits provide a clear exception to this rule. It is thus understandable that early rabbit domesticators selectively bred relatively calm individuals, reflected in these genetic changes. Charles Darwin himself (who discussed domestication extensively in his Origin of Species) noted that “hardly any animal is more difficult to tame than the young of the wild rabbit; scarcely any animal is tamer than the young of the tame rabbit”.
One thing Andersson’s team did not find was a single example of a gene having been inactivated during domestication. Andersson personally doubts the existence of so-called ‘domestication genes’, suggesting that domestication relies not on major changes to a few genes but is rather a polygenic trait, relying on changes in the frequencies of many alleles.
The curious incident of the seal in the pond
A curious case of an apparent historical novelty supporting a modern developmental theory was the topic of a talk by Yoland Savriama of the University of Helsinki. In Stockholm in 1929, a ringed seal and grey seal bred in captivity and produced a hybrid offspring. While this offspring was dead at birth, that the two species could breed at all is remarkable; as Savriama pointed out, the two seal species are four times more diverse than, for example, humans and Neanderthals.
The skull of this hybrid was preserved and, almost 100 years later, Savriama and colleagues used geometric morphometrics to compare its skull with those of pure-bred ringed and grey seals.
The overall phenotype of the hybrid skull was an almost perfect intermediate of its two parent species. Closer analysis however, suggested that rather than being a blend of its two parent species, different regions of the hybrid skull were more closely aligned to one species than the other. While of course noting the caveat of this analysis being based on a single sample, Savriama believes these findings support the idea of modularity in the development of seal skulls.
Modules in this concept are broadly defined as independent units which while strongly coherent within themselves, are independent from the other modules making up the larger morphological whole. So in this case, the hybrid offspring inherited different modules from each of its parental species; the individual skull modules show clear evidence of the species they were inherited from, with only the overall skull appearing as an intermediate of the two species.
Of placentas, primates and marsupials
Modularity was a popular topic throughout the conference, including in a symposia on the role of developmental evolution in reproductive medicine. Here though, the question was how to apply developmental theories like modularity to more amorphous morphologies.
Mick Elliot, of Cambridge University, noted this problem in his talk on applying evolutionary ideas to the human placenta. Modularity is a relatively easy concept when you are looking at clearly segmented animals like the worms so often used as model species in developmental biology; but how do you define modules in an organ like the placenta? It is also hard to determine homology – the shared ancestry of structures in different species – when placental morphology varies so considerably among mammals that working out what parts are equivalents between species becomes very difficult.
Despite these difficulties, Elliot stressed the importance of efforts to widen our knowledge of pregnancy-related medical conditions beyond the usual model species. As he noted, the most widely used model species to study preeclampsia is the mouse – despite this disease not actually occurring in mice! Meanwhile, as both Elliot and Derek Wildman in another talk discussed, we still know close to nothing about how common (or non-existent) preeclampsia is in our primate relatives.
The need to expand our range of model species was one of the themes of the symposia. Marilyn Renfree of the University of Melbourne told of her group’s success using marsupials as model species. By virtue of most of their development taking place after birth, marsupials present a unique opportunity for developmental research. The same phases of development that take place in utero in eutherian mammals (thus preventing manipulation of the young without killing them), in marsupials takes place in the mother’s pouch, allowing manipulation of the young without harm.
Renfree described her use of the Tammar Wallaby as a model to investigate the development of sex disorders, such as micro-penises, in humans. At birth, wallaby gonads have yet to develop into male or female genitalia. Thus by treating them with varying levels of hormones like androgens (delivered orally, like feeding a baby with a bottle), Renfree’s group have begun to understand the effect of variations in hormone levels on genital development.
Vestiges of the future for ants
The conference was concluded with a keynote talk by Ehab Abouheif of McGill University entitled “Dear ants, what have you done for Evo-devo?” So, what have the ants done? Well, Abouheif noted that the substantial morphological differences between the genetically identical queens and workers (differences entirely created by the food fed to them as larvae) and between sub-castes of workers in many species, are intrinsically interesting to evolutionary developmental biologists.
However the focus of Abouheif’s talk was on one particular aspect of ant morphology: wings. Queens (before they mate) and males (who die immediately after mating) have wings, but ant workers do not. Workers do still maintain vestigial wing discs however, a universal feature of all ant species.
For 20 years, Abouheif (like all other researchers) assumed these wing discs had no function, merely being one of the many vestigial body parts we see in nature, evidence of natural selection but nothing more. Recently however, his lab has discovered this is not the case; the wing discs can have a major impact on ant development and, potentially, on ant evolution.
Working on the ant genus Pheidole that produces soldier and worker sub-castes, Abouheif found that knocking out the gene linked to wing discs interferes with the development of the much larger head size of the soldiers. In a sense, development of larvae into soldiers is ‘activated’ by the wing discs, via hormone levels.
Even more excitingly, the wing discs are linked to the development of novel castes. Some Pheidole species produce ‘super-soldiers’ with extremely large heads; these they use to block nest entrances, helping protect their colony when under attack by marauding army ants. Once again, the development of these extra-large heads is linked to the vestigial wing discs.
Abouheif and his lab members have even spotted what appear to be random ‘super-soldier’ individuals in species that do not actually produce super-soldiers (and would have no apparent need to, living in northern America where predatory army ants are not found). It appears these are produced by random mutations in the wing disc genes; an example of the randomly generated variation that provides the raw material for natural selection.
What appeared to be nothing more than the vestiges of a characteristic lost by worker ants millions of years ago, in fact turns out to be the source of new variation leading to novel evolutionary innovations.