The Mouse Lemur, a Genetic Model Organism for Primate Biology, Behavior, and Health

Abstract

Systematic genetic studies of a handful of diverse organisms over the past 50 years have transformed our understanding of biology. However, many aspects of primate biology, behavior, and disease are absent or poorly modeled in any of the current genetic model organisms including mice. We surveyed the animal kingdom to find other animals with advantages similar to mice that might better exemplify primate biology, and identified mouse lemurs (Microcebus spp.) as the outstanding candidate. Mouse lemurs are prosimian primates, roughly half the genetic distance between mice and humans. They are the smallest, fastest developing, and among the most prolific and abundant primates in the world, distributed throughout the island of Madagascar, many in separate breeding populations due to habitat destruction. Their physiology, behavior, and phylogeny have been studied for decades in laboratory colonies in Europe and in field studies in Malagasy rainforests, and a high quality reference genome sequence has recently been completed. To initiate a classical genetic approach, we developed a deep phenotyping protocol and have screened hundreds of laboratory and wild mouse lemurs for interesting phenotypes and begun mapping the underlying mutations, in collaboration with leading mouse lemur biologists. We also seek to establish a mouse lemur gene "knockout" library by sequencing the genomes of thousands of mouse lemurs to identify null alleles in most genes from the large pool of natural genetic variants. As part of this effort, we have begun a citizen science project in which students across Madagascar explore the remarkable biology around their schools, including longitudinal studies of the local mouse lemurs. We hope this work spawns a new model organism and cultivates a deep genetic understanding of primate biology and health. We also hope it establishes a new and ethical method of genetics that bridges biological, behavioral, medical, and conservation disciplines, while providing an example of how hands-on science education can help transform developing countries.

Keywords: Madagascar; Microcebus; model organism; mouse lemur; primate genetics.

Figure 1

Phylogenetic tree showing evolutionary relationship of primates and other vertebrates. Branch length (horizontal distance) reflects the mutational load in coding and noncoding sequences for a species. Note that rodents (rat, mouse) diverged from the primate lineage more recently than all other animals shown, but their rate of mutation accumulation (mutational clock) is several times greater than that of other vertebrates, as indicated by the long cumulative branch length since separation of mouse and rat from the primate lineage. Genetic distance between species can be estimated from the sum of the branch lengths connecting them, which is about two times greater from mouse to human than from mouse lemur to human. Phylogenetic tree adapted from Margulies et al. (2007).

Figure 2

Candidates for a genetic model organism that could better mimic primate biology than mouse. Animals shown are small mammals (<200 g) that diverged more recently from the primate lineage than mouse. (A) Northern tree shrew T. belangeri (∼150 g), a mammal found in southeast Asia. (B) Bush baby G. demidovii (∼60 g), a prosimian (strepsirrhine) primate found in west and central Africa. (C) Spectral tarsier T. spectrum (∼120 g), a prosimian primate (recently grouped with haplorrhines) found in Southeast Asia. (D) Pygmy marmoset C. pygmaea (∼120 g), the world’s smallest monkey (haplorrhine), found in the Amazon basin of South America. Photo credits: (A) http://www.philadelphiazoo.org, (B) http://www.arkive.org, (C) http://www.ecologyasia.com, and (D) http://www.justviral.eu.

Figure 3

Field study of mouse lemurs. (A) Geographical distribution of mouse lemurs in Madagascar (off east coast of mainland Africa, boxed in inset) and major field study sites (filled ○). Capital city Antananarivo (○) is located centrally, inland from coastal forested areas (green shading). The approximate distribution of 11 of the >20 described species of mouse lemurs is shown by color outlines as indicated in the legend; most species are “microendemic” to a region delimited by rivers, mountains, or elevation gradients. Field study sites include: RNP (rain forest), Kirindy Mitea National Park (dry deciduous forest), Ankarafantsika Nature Reserve (dry tropical forest), Andohahela National Park (spiny forest), and Beza Mahafaly Reserve (gallery forest). (B) Aerial view of CVB research station at RNP, established by Patricia Wright and colleagues in 1991 (RNP) and 2003 (CVB). The state-of-the art interdisciplinary research station (http://www.stonybrook.edu/commcms/centre-valbio/) focuses on the unique flora and fauna of the surrounding rain forest, and includes a full molecular and cell biology laboratory and conference center where ecologists and conservation biologists interact with genetics, health, and engineering experts as well as educators and artists. (C) A brown mouse lemur, M. rufus, foraging at night near CVB. Mouse lemurs are nocturnal so field studies are done by capture and release in the evening when they awaken. (D) Field researchers inspecting an aluminum Sherman live trap baited with banana to attract mouse lemurs. (E) A captured brown mouse lemur brought back to the laboratory. An identifying microchip is implanted and the animal thoroughly examined (“deep phenotyping”) before its release back into the wild at the capture site the same evening. Individuals are studied longitudinally by periodic recapture throughout their up to 10 year life span. (F) Scientist examining mouse lemur fibroblast culture in CVB laboratory. Map data from Google (copyright 2016). Microcebus spp. geographical distributions adapted from International Union for Conservation of Nature Red List of Threatened Species (http://www.iucnredlist.org/). Photo credits: (B) Khen Randriamamonjy, (C–E) Guy Albertelli.

Figure 4

Laboratory study of mouse lemurs. Mouse lemurs have been studied in the laboratory since establishment of a breeding colony of gray mouse lemur M. murinus by J.-J. Petter and A. Petter-Rousseaux in the 1960s from a dozen individuals imported from southwest Madagascar, like the individual shown foraging in the dry deciduous forest of Kirindy Mitea National Park (A). These tiny strepsirrhine primates are docile (B) and readily maintained in enriched cages with branches or tubes simulating their arboreal habitat, and nest boxes (C) simulating the tree holes where they typically sleep in small groups (D). Photo credits: Guy Albertelli (A, C, and D).

Figure 5

High school students exploring the unique biology outside their school. (A) Lycée Kelilalina, a new high school in a small rural village near CVB field station. (B) Like most schools in Madagascar, it has a “living laboratory” right outside the door, ripe for exploration. (C) Students initializing a catch-and-release field study on mouse lemurs around the school. (D–F) Students assembling powerful paper microscopes (Foldscopes, http://www.foldscope.com) to explore the microscale biology surrounding the school.

Figure 6

“Darwin’s finches” of the primates. (A) Phylogenetic tree of the ∼100 species of lemurs, all endemic only to Madagascar. Parentheses, extinct species. In the ∼60 MY since their common ancestor colonized the island, lemurs radiated into diverse forms and functions adapted to nearly every ecological niche, like Darwin’s finches did in the Galapagos Islands. (B–E) The lemur clade includes (B) the iconic ring-tailed lemur L. catta featured as King Julien XIII in the Dreamworks movie Madagascar, (C) the greater bamboo lemur Prolemur simus that can metabolize the cyanide in ingested bamboo, (D) the red-fronted brown lemur Eulemur rufifrons with its distinctive sexually dichromatic coat, and (E) the indri Indri indri with its bellowing song that can be heard a kilometer away. Like most of Madagascar’s legendary endemic flora and fauna, all lemurs are critically threatened by deforestation, although gray mouse lemurs have International Union for Conservation of Nature “least concern” status because of their abundance. Phylogenetic tree adapted from Herrera and Davalos (2016). Photo credits: (B–E) Guy Albertelli.