Mouse lemurs are the smallest, fastest reproducing, and among the most abundant primates, and an emerging model organism for primate biology, behavior, health and conservation. Although much has been learned about their physiology and their Madagascar ecology and phylogeny, little is known about their cellular and molecular biology. Here we used droplet- and plate-based single cell RNA-sequencing to profile 226,000 cells from 27 mouse lemur organs and tissues opportunistically procured from four donors clinically and histologically characterized. Using computational cell clustering, integration, and expert cell annotation, we defined and biologically organized over 750 mouse lemur molecular cell types and their full gene expression profiles. These include cognates of most classical human cell types, including stem and progenitor cells, and the developmental programs for spermatogenesis, hematopoiesis, and other adult tissues. We also described dozens of previously unidentified or sparsely characterized cell types and subtypes. We globally compared cell type expression profiles to define the molecular relationships of cell types across the body, and explored primate cell type evolution by comparing mouse lemur cell profiles to those of the homologous cells in human and mouse. This revealed cell type specific patterns of primate cell specialization even within a single tissue compartment, as well as many cell types for which lemur provides a better human model than mouse. The atlas provides a cellular and molecular foundation for studying this primate model organism, and establishes a general approach for other emerging model organisms.

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.

The gray mouse lemur (Microcebus murinus, GML) is a nocturnal, arboreal, prosimian primate that is native to Madagascar.Captive breeding colonies of GMLs have been established primarily for noninvasive studies on questions related to circadian rhythms and metabolism. GMLs are increasingly considered to be a strong translational model for neurocognitive aging due to overlapping histopathologic features shared with aged humans. However, little information is available describing the clinical presentations, naturally occurring diseases, and histopathology of aged GMLs. In our colony, a 9 y-old, male, GML was euthanized after sudden onset of weakness, lethargy, and tibial fracture. Evaluation of this animal revealed widespread fibrous osteodystrophy (FOD) of the mandible, maxilla, cranium, appendicular, and vertebral bones. FOD and systemic metastatic mineralization were attributed to underlying chronic renal disease. Findings in this GML prompted periodic colony-wide serum biochemical screenings for azotemia and electrolyte abnormalities. Subsequently, 3 additional GMLs (2 females and 1 male) were euthanized due to varying clinical and serum biochemical presentations. Common to all 4 animals were FOD, chronic renal disease, uterine adenocarcinoma (females only), cataracts, and osteoarthritis. This case study highlights the concurrent clinical and histopathologic abnormalities that are relevant to use of GMLs in the expanding field of aging research.

Physical performance is crucial for animal survival and fitness. In this context, greater bite forces can provide advantages and may allow an individual to gain access to reproductive partners and/or different food resources. Here, we explored the determinants of bite force in a wild population of the brown mouse lemur (Microcebus rufus). Our objectives were to elucidate (1) if sex, head width, heart rate (as an indicator of overall physical fitness) and body condition drive variation in bite force in this population of wild mouse lemurs; and (2) the relative importance of the ecological niche in determining bite force by comparing results from this wild population with previously published results on bite force, body mass and head width from a laboratory colony of the grey mouse lemur (Microcebus murinus). We captured 32 wild brown mouse lemurs at night in the Ranomafana National Park in Madagascar during the beginning of the rainy season from 1st to 31st October 2016. We measured bite force, heart rate, body mass and head width of all individuals, and assigned sex and body condition (estimated as the unstandardized residual of a regression of body mass against head size). Although maximum bite force was positively correlated with body mass, it was not correlated with body condition. Residual bite force was highly correlated with residual head width and heart rate. The mean bite force of wild brown mouse lemurs was much lower than that of grey mouse lemurs in captivity, but showed similar relationships to head dimension and body mass. Even when corrected by body condition, grey mouse lemurs bit significantly harder than brown mouse lemurs. The difference in bite force between species could be explained by differences in head size and niche divergence with brown mouse lemurs eating mostly soft fruits and grey mouse lemurs eating more hard insects.