Newswise — There has been considerable interest in the publication of the platypus genome but it has led to a lot of confusion and some misconceptions about evolutionary relationships.
Platypus Sex Chromosomes And Basal-Equals-Primitive by Ryan Gregory.PhD
While most mammals show an XY system (males XY, females XX), the monotreme system is quite unique (5X5Y males, 5X5X females), and has some similarities to birds in terms of which genes are involved in determining sex (1).
Thus, because monotremes have a system quite different from therians (placental and marsupial mammals) and the split between monotremes and therians occurred earlier than that between placentals and marsupials, the system in therians must have evolved after the split between theirs and the monotreme lineage. Also, one can identify regions on platypus autosomes that correspond to the sex chromosomes in other mammals, suggesting that the therian sex determining genes are derived from former autosomal regions (this was already apparent through comparisons with other non-mammalian vertebrates).
The problem is the way this has been framed. The platypus is a modern species. Like all modern species, it retains some ancestral characters (e.g., egg-laying and milk secretion without nipples), posseses many highly derived characters (e.g., electroreception and venom), and shows convergent evolution with some other groups (e.g., "duck bill"). It is entirely possible that the chromosomal system in platypus is similar to the one found in the common mammalian ancestor, but to assume that because this species is "basal" (i.e., "early branching") it must be primitive in terms of any particular characteristic is a fallacy.
What The Platypus Genome Is and Isn't by Michael White, PhD
Is the platypus part bird, part reptile part mammal, an amalgam of very different groups of animals? Is it a primitive mammal that resembles the early ancestors of all mammals? Can we figure out just what this creature is by gazing at its genome?
The platypus Ornithorhynchus anatinus is part of a group of mammals known as Monotremes, who last shared a common ancestor with us between 160 and 200 million years ago (2). At that point, one mammalian line split off to give rise to today's monotremes, while another eventually led to the marsupial and placental mammals. What this means, of course, is that the platypus is as distant from that ancient ancestor as we are - humans, kangaroos, and platypuses are all modern mammals.
However some traits of the platypus can be considered primitive, if you take primitive to mean 'resembles the ancestral form' (instead of the more colloquial meaning 'less advanced'). The most obvious example is that the platypus lays eggs, something which ancient mammals also did, but which most modern mammals no longer do. Egg-laying is an ancestral trait that has persisted in this particular evolutionary line, a trait which the platypus inherited from the egg-laying, reptile-like ancestors of all mammals - in that sense, it's primitive.
Other features of the platypus aren't primitive, and have absolutely nothing to do with the fact that all mammals shared a common ancestor with today's birds and reptiles. The venomous spines possessed by the male platypus are not there because of some reptile ancestor that had venomous spines; early mammals probably had no venom, and evolutionary biologists have long insisted that this interesting platypus feature is likely an independent evolutionary invention. Venomous spurs are no more reptilian than the human lack of fur: we don't have fur, and our reptile ancestors didn't have fur, but our bare skin is not a trait inherited from fur-less reptile-like ancestors, it's a trait we lost recently in our evolutionary history.
(1) http://www.scientificblogging.com/genomicron/platypus_sex_chromosomes_and_basal_equals_primitive
(2) http://www.scientificblogging.com/adaptive_complexity/what_the_platypus_genome_is_and_isnt
Scientific Blogging is the world's largest online science community, with renowned featured writers, academics and scientists from the private sector writing on the latest developments in science.
RSS