Two new studies (1, 2) highlight the remarkable genetic diversity of lemurs in Madagascar. The small nocturnal primates were the first placental mammals to arrive on the island some 50 million years ago. They had no known predators and little competition, which may explain the rapid expansion into at least a hundred living species.
The two new lemur species were first captured by co-author Rodin Rasoloarison, a researcher at the University of Antananarivo in Madagascar, during trips into the forest habitats in 2003 and 2007. Rasoloarison and colleagues used two mitochondrial and four nuclear genes to place the specimens they captured in the mouse lemur phylogenic tree. Those results were published in 2010 (3), but the current paper formally describes them as new species.
In a phylogenic tree, long branches reflect long periods of reproductive isolation and genetic divergence, but that isn’t sufficient to declare two populations to be separate species. Researchers must also link genetic differences to morphological differences. Home ranges are also taken into account, although two populations in the same range may be different species if some other reproductive barrier is present. “You can’t do it on the basis of DNA (alone). It’s the combination (of genetics, morphology, and geographic range) that allows us to make these (decisions),” said Anne D. Yoder, professor of biology at Duke University and director of the Duke Lemur Center who was an author on the mouse lemur paper.
The mouse lemurs remain challenging to identify because they are nocturnal and separate species look so much alike. To determine a new species, “You have to do the meticulous morphological analysis and the genetic work as well, but (the twenty species) are real. We’re not making this up,” Yoder said.
Discovering a new species is exciting, but in this case it was also disheartening. When Rasoloarison returned to the lemur habitats last year, he found the region inhabited by one of the newly identified species, the Marohita mouse lemur, obliterated by deforestation. “I’m not sure the species is extinct, but they are certainly on the edge of it. That’s a crushing kind of conclusion,” said Yoder.
Yoder’s next project is to determine what drives the evolution of new lemur species. The animals are so diverse that reproductive isolation is not likely to be caused by geographical isolation alone. She believes it may come down to mate selection. “They’re using some sensory criteria to decide if they want to mate with another individual. It’s not vision because they’re nocturnal. It would have to be auditory or olfactory,” said Yoder. Her team is analyzing olfactory genes in hopes of finding variants that track with speciation.
The second study, published in Proceedings of the National Academy of Sciences, focused on a single lemur species known as the aye-aye. Researchers sequenced the genomes of twelve individuals from the northern, eastern, and western regions of Madagascar. Eastern and western aye-ayes varied somewhat from one another, but the northern population was particularly genetically distinct, suggesting that it had been reproductively isolated for a long time.
To get a sense of the genetic distance between aye-aye populations, researchers took the additional step of comparing the distance between the northern and east-west populations to the distance between present-day human groups. They compared 12 human DNA sequences from African agriculturalists, Europeans, and Southeast Asians. The African and European human populations are more genetically similar to one another than the northern and east-west aye-aye populations, which are separated geographically by just 160 miles, although that barrier includes high plateaus and large rivers.
Both studies highlight the extraordinary genetic diversity of lemurs and pose challenges for conservationists seeking to maintain the animal’s rich species diversity. This is particularly urgent since lemurs are the most endangered mammals in the world, according to the International Union for the Conservation of Nature.
And let’s not forget the appeal of those big eyes. “They’re just glorious creatures,” said Yoder.
1. R.M. Rasoloarison, D.W. Weisrock, A.D. Yoder, D. Rakotondravony, P. M. Kappeler. Two New Species of Mouse Lemurs (Cheirogaleidae: Microcebus) from Eastern Madagascar. International Journal of Primatology, published 2013 Mar 27. [Epub ahead of print] doi: 10.1007/s10764-013-9672-1
2. G.H. Perry, E.E. Louis, Jr., A. Ratan, O.C. Bedoya-Reina, R.C. Burhans, R. Lei, S.E. Johnson, S.C. Schuster, W. Miller. Aye-aye Population Genomic Analyses Highlight an Important Center of Endemism in Northern Madagascar. Proceedings of the National Academy of Sciences, 2013 Mar 25. [Epub ahead of print] doi:10.1073/pnas.1211990110
3. D.W. Weisrock, R.M. Rasoloarison, I. Fiorentino, J.M. Ralison, S.M. Goodman, P.M. Kappeler, A.D. Yoder (2010). Delimiting species without nuclear monophyly in Madagascar’s mouse lemurs. PLoS One, 5, e9883. doi:10.1371/journal.pone.0009883