Time for look at another paper. This one’s on something I think I referred to earlier – the use of ancient DNA to determine the sex of New Zealand’s giant, flightless – & alas! extinct – moa.
This technique relies on the fact that birds, like mammals, have sex chromosomes. In mammals these are the X and Y chromosomes (males have XY, & females XX). In birds, it’s ZW for females and ZZ for males. (The state of having 2 different sex chromosomes is described as ‘heterogametic’.) Huynen et al. demonstrated that W chromosome markers could be used to determine the sex of individuals from the various species of living ratites (emu, ostrich, rhea, cassowary, kiwi, & tinamous), & went on to apply this test to the moa.
Moa presented an interesting problem, because of all the available fossil material, only two specimens were known to be female. Those fossils show considerable variation in size, within species & between species, & there are at least 2 competing hypotheses to explain this. On the one hand, it’s been suggested that the variation in size indicated the presence of pairs of species, one large-bodied & one small. Alternatively, it could be that in at least some moa species the size variation indicates strong sexual dimorphism, with one sex being markedly larger than the other.
Huynen & the team tested this second hypothesis. Having sequenced the W chromosome from several moa species, they designed PCR primers that were tested on the two known female moa specimens – & sexed them correctly. The next step was to sex 115 individual moa. (These fossils had radiocarbon ages ranging from 1,319 – 6,227 years before present, which shows it’s possible to obtain nuclear DNA sequences from relatively ancient bones. mtDNA tends to last longer, because it starts off with a much higher copy number.)
They found that moa were characterised by extreme reverse sexual dimorphism, with females the larger sex. (This isn’t actually all that uncommon – think of spiders, for example. Or eagles, & other predatory birds. Or – taken to extremes – in some species of angler fish, where the tiny male lives parasitically on the female’s body. It’s just that we mammals tend to think that everything follows the same pattern that we do). This allowed them to add to our understanding of just how many species of moa there were: while early analyses, based on skeletons, described 20 genera and at least 64 species, more recent estimates have lowered the species number to 38 and then to 11. Take the largest moa genus, Dinornis, as an example. Prior to this research project there were 3 named species of Dinornis: giganteus, novaezealandiae, & struthoides. Huynen & his co-workers were surprised at what they found:
[the] single North Island femur of D. giganteus and the material from the seven North Island D. novaezealandiae examined were from females. Similarly, South Island D. giganteus and most D. novaezealandiae were females, and in both islands all specimens of D. struthoides examined were males.
Their conclusion: there were only 2 species of Dinornis, one in the North Island and one in the South. And, using femur length as an indicator of body size, in this species females were almost twice the mass of their male counterparts.
(I bet I know who won the arguments in those families 😉 )
L.Huynen, C.D. Millar, R.P. Schofield & D.M. Lambert (2003) Nuclear DNA sequences detect species limits in ancient moa. Nature 425: 175-178