Crocodile immunity remains down the ages

Although crocodiles and alligators diverged from the same reptilian ancestor 90 million years ago, the immune systems of both have remained relatively unchanged despite their worldwide distribution, according to a global study of the reptiles.

The researchers say that alligators and crocodiles occupy an evolutionary mid-point between mammals and birds and so provide a unique link between the two classes of animals. They say the research helps address fundamental questions about how evolution drives and maintains genetic diversity within immune genes.

The scientists looked at the diversity and evolutionary mechanisms of two primary gene classes of key components of the reptiles’ immune system, the major histocompatibility complex or MHC. They investigated the genetic make-up of 20 species of the crocodilians, including US alligators and the two Australian crocodile species of freshwater and saltwater.

The scientists say the research helps to close a gap in their knowledge of immune gene evolution, particularly since the crocodilian families – crocodiles and alligators – diverged from their common ancestor 90 million years ago.

The MHC is a group of genes that help the immune system identify microbes and parasites. They play an important role in disease resistance because diverse genes allow animals to resist a wider range of diseases.

The research, published in the journal PLOS ONE, found that some of the genes involved in the fight against viruses, bacteria and parasites have remained the same across all crocodilian species while other immune genes seem to have diversified in crocodiles.

The scientists say the diverse environments occupied by many crocodilians, whether Australian saltwater crocodiles or Florida alligators appear to have exposed the crocodilians' immune genes to a wide range of germs.

Researchers found multiple instances of crocodilians losing or duplicating genes, showing their immune systems were still responsive to evolutionary changes.

“We now have a genetic resource to understand the immune system in crocodilians, thanks to this research. It will enable genetic investigations of how these animals respond to local conditions including susceptibility to disease,” said lead author of the paper Dr Weerachai Jaratlerdsiri.

“In an agricultural context, crocodiles are produced for their skins as part of a very successful sustainable-use conservation strategy. Part of this strategy is to place an economic value on the wild population, in this case the crocodile eggs, which are collected and artificially incubated before rearing the offspring in captivity. “

The scientists say that because crocodiles and alligators are not domesticated, no selection against viruses and bacteria has occurred. As a result, understanding the genetic regulation of disease susceptibility will provide crocodile producers with selection tools and lessen the reliance on vaccinations and antibiotics.

“The innovative and fundamental knowledge generated from this research serves as the base for further research into the immunological fitness of wild and farmed populations especially to explain how they maintain the health of their immune system to deal with parasites and microbes,” the researchers say.

“We suggest that throughout crocodilian evolution, immune gene diversity responded to disease-causing organisms in the environment. This might provide further insights into disease resistance by explaining how immune genes evolve in other vertebrates, in particular reptiles.”