Crocodiles on My MindPosted: September 19, 2011
I recently spent a Sunday evening at home watching the Nature episode entitled “Supersize Crocs” on PBS. It features an account of herpetologist Romulus Whitaker’s search for the last remaining giant crocodiles on Earth. Rom has been studying crocs for decades, and has worked tirelessly for their conservation (you can read his Wikipedia page here). Giant crocs were once hunted relentlessly worldwide for their skins. The larger crocs were obviously worth more money, and by the 1970s most large crocs were shot out. Most croc species have been protected since then, and Whitaker wondered if 30 to 40 years of conservation measures was enough time for a new generation of giant crocs to grow. In a search spanning three continents of Africa, Asia and Australia, Rom found terrific evidence for 17 and 18 foot crocs, but alas no proven 20-footers.
Rom sure missed a doozy, because in August 2011 a possibly record-shattering 21 foot long croc was captured in the Phillipines, making international headlines. The remarkable specimen is an estuarine or saltwater crocodile (Crocodylus porosus), known to be the largest living species of reptile.
Crocodiles have fascinated me since childhood, when I learned they were among the closest relatives of dinosaurs. As one of the last remaining members of the once great reptilian group Archosauria, modern crocodiles (and their relatives, alligators and caimans) have adopted an extremely successful strategy of aquatic predation, and are supremely adapted to their ecological niche as top predators in many tropical and subtropical environments across Africa, Asia, Australia, and the Americas.
A layperson may think all crocodiles are the same, however this is not the case as many of the modern lineages are separated by millions of years of divergent evolution. Two recent studies use molecular phylogenetic techniques in order to obtain a well-resolved picture of how the living croc species are related to one another.
Meganathan PR, Dubey B, Batzer MA, Ray DA, Haque I. (2011) Complete mitochondrial genome sequences of three Crocodylus species and their comparison within the Order Crocodylia. Gene. 2011 Jun 1;478(1-2):35-41.
This study corroborates several already-existing hypotheses about crocodile evolution. Mainly, that crocodiles are monophyletic, meaning members of the genus Crocodylus all descended from a single ancestor. Also, that the two living species of gharials (fish-eating Asian crocs with slender snouts) are also monophyletic and form a group separate from other crocodiles.
The uniqueness of this paper is in the fact that it used whole mitochondrial genome sequences to reconstruct the branching pattern of croc evolution. Why use complete mitochondrial genome sequences? The mitochondrial genome is made of a few genes, and each of them evolve at a slightly different rate. That means that observed divergences may differ depending on which gene you are using to track the evolution of a group of organisms. Using all the genes in the mitochondrial genome provides a much greater chance of recovering the true branching pattern and rate of evolution.
However, whole mitogenomic sequencing is relatively expensive and resource-intensive. These kinds of studies are therefore at risk of sampling bias, because they often only include one member of each species. Another recent croc paper highlights the utility of sampling more than one member of a species.
Robert W. Meredith, Evon R. Hekkala, George Amato, John Gatesy. (2011) A phylogenetic hypothesis for (Crocodylia) based on mitochondrial DNA: Evidence for a trans-Atlantic voyage from Africa to the New World. Molecular Phylogenetics and Evolution. Volume 60, Issue 1, July 2011, Pages 183-191
This is a paper that will certainly make a giant-croc-sized splash because it forces new thinking about the iconic and revered (literally, by the ancient Egyptians) Nile crocodile. Using the tools of molecular phylogenetics, again by focusing on the divergences of mitochondrial genes, the authors demonstrate that Crocodylus niloticus is a paraphyletic species. This means that some populations of Nile crocs are more closely related to other species of crocodiles. Since the goal of modern systematics and taxonomy is to ensure that species designations reflect proper evolutionary relationships (biologists call this “establishing the reciprocal monophyly of species”), the consequences are that this species will be split into two, and they suggest a new species Crocodylus suchus. Even more interestingly, these “formerly-known-as-Nile-crocs” of East Africa group with C. moreletii, C. acutus, and C. rhombifer, which are American crocs.
The authors assert that this suggests an African crocodile species colonized the New World, leading to the formation of the American croc species we see today (“out of Africa” hypothesis for American crocs, if you will…). How would a crocodile get from Africa to America, you may ask? Why, it would swim, of course! The authors therefore propose that an ancient trans-Atlantic voyage took place. The precise date of this journey is not given, yet they take a stab at it using evidence from the fossil record. The oldest fossils of any Crocodylus date to the late Miocene (the geologic epoch that ended ~5 million years ago), while the oldest known fossils of C. niloticus in Africa and all American crocs date to the Pliocene (which lasted ~5 to ~2.5 million years ago). Therefore, the authors believe that the New World croc migration occurred relatively recently, when the span of the Atlantic Ocean was hundreds of miles wide! This not an outlandish proposition, as crocs are known to inhabit the ocean (hello, saltwater croc), where they are capable of long-distance dispersal. In fact, crocs are known to “island hop” by body surfing.
One thing about the Meredith et al. study and how much weight we should put into the authors’ interpretations: even though they used evidence from multiple genes within the mitochondria, these genes are usually inherited as a single unit because genetic recombination and horizontal gene transfer should be very rare in the mitochondrial genome. This means that evolutionary trees generated from mitochondrial genes should really be interpreted as gene trees, and not necessarily species trees (the authors of Meredith et al. do in fact admit to this). In order to obtain a well-supported species tree from genetic data, it behooves investigators to use genes from the mitochondria as well as multiple genes from the nuclear genome. This is because each gene genealogy will have a slightly different topology due to stochastic effects of coalescence. Coalescent theory is a mathematically intensive yet surprisingly intuitive field of population genetics that follows the genealogy of genes in a population backwards in time. We don’t have to go into it here, but it has revolutionized the field of phylogenetics by making biologists think more about the theoretical and practical differences between species trees and gene trees.
Therefore, more work is definitely needed if the intriguing Meredith et al. conclusions are going to become adopted by the greater scientific community. Nonetheless, the prospect of a seafaring crocodilian pioneer braving the vast ocean to colonize a new land is an exciting one. And it got me thinking about crocs this week, which is always a good thing.
- Crocodile bites human ancestor (boingboing.net)
- Nile crocodile is actually two species (and the Egyptians knew it) | Not Exactly Rocket Science (blogs.discovermagazine.com)
- World’s largest crocodile caught in Philippines (with video) (theprovince.com)