Chromosomal evolution in Malagasy lemurs. V. Chromosomal banding studies of Lemur fulvus albifrons, Lemur rubriventer and its hybrids with Lemur fulvus fulvus

Ecology and age, but not genetic ancestry, predict fetal loss in a wild baboon hybrid zone

OBJECTIVES

Pregnancy failure represents a major fitness cost for any mammal, particularly those with slow life histories such as primates. Here, we quantified the risk of fetal loss in wild hybrid baboons, including genetic, ecological, and demographic sources of variance. We were particularly interested in testing the hypothesis that hybridization increases fetal loss rates. Such an effect would help explain how baboons may maintain genetic and phenotypic integrity despite interspecific gene flow.

MATERIALS AND METHODS

We analyzed outcomes for 1020 pregnancies observed over 46 years in a natural yellow baboon-anubis baboon hybrid zone. Fetal losses and live births were scored based on records of female reproductive state and the appearance of live neonates. We modeled the probability of fetal loss as a function of a female's genetic ancestry (the proportion of her genome estimated to be descended from anubis [vs. yellow] ancestors), age, number of previous fetal losses, dominance rank, group size, climate, and habitat quality using binomial mixed effects models.

RESULTS

Female genetic ancestry did not predict fetal loss. Instead, the risk of fetal loss is elevated for very young and very old females. Fetal loss is most robustly predicted by ecological factors, including poor habitat quality prior to a home range shift and extreme heat during pregnancy.

DISCUSSION

Our results suggest that gene flow between yellow and anubis baboons is not impeded by an increased risk of fetal loss for hybrid females. Instead, ecological conditions and female age are key determinants of this component of female reproductive success.

Primate comparative genomics: lemur biology and evolution

Comparative genome sequencing projects are providing insight into aspects of genome biology that raise new questions and challenge existing paradigms. Placement in the phylogenetic tree can often be a major determinant of which organism to choose for study. Lemurs hold a key position at the base of the primate evolutionary tree and will be highly informative for the genomics community by offering comparisons of primate-specific characteristics and processes. Combining research in chromosome evolution, genome evolution and behavior with lemur comparative genomic sequencing will offer insights into many levels of primate evolution. We discuss the current state of lemur cytogenetic and phylogenetic analyses, and suggest how focusing more genomic efforts on lemurs will be beneficial to understanding human and primate evolution, as well as disease, and will contribute to conservation efforts.

Characterization of a highly repeated DNA sequence family in five species of the genus Eulemur

The karyotypes of Eulemur species exhibit a high degree of variation, as a consequence of the Robertsonian fusion and/or centromere fission. Centromeric and pericentromeric heterochromatin of eulemurs is constituted by highly repeated DNA sequences (including some telomeric TTAGGG repeats) which have so far been investigated and used for the study of the systematic relationships of the different species of the genus Eulemur. In our study, we have cloned a set of repetitive pericentromeric sequences of five Eulemur species: E. fulvus fulvus (EFU), E. mongoz (EMO), E. macaco (EMA), E. rubriventer (ERU), and E. coronatus (ECO). We have characterized these clones by sequence comparison and by comparative fluorescence in situ hybridization analysis in EMA and EFU. Our results showed a high degree of sequence similarity among Eulemur species, indicating a strong conservation, within the five species, of these pericentromeric highly repeated DNA sequences.

Kinetochore reproduction in animal evolution: cell biological explanation of karyotypic fission theory

Karyotypic fission theory of Todd offers an explanation for the diverse range of diploid numbers of many mammalian taxa. Theoretically, a full complement of acrocentric chromosomes can be introduced into a population by chromosomal fission. Subsequent inheritance of ancestral chromosomes and paired fission derivatives potentially generates a diploid range from the ancestral condition to double its number of chromosomes. Although it is undisputed that both chromosomal fission and fusion ("Robertsonian rearrangements") have significantly contributed to karyological diversity, it is generally assumed that independent events, the fission of single chromosomes or the fusion of two chromosomes, are the sources of such change. The karyotypic fission idea by contrast posits that all mediocentric chromosomes simultaneously fission. Here I propose a specific cell biological mechanism for Todd's karyotypic fission concept, "kinetochore reproduction theory," where a complete set of dicentric chromatids is synthesized during gametogenesis, and kinetochore protein dephosphorylation regulates dicentric chromatid segregation. Three postulates of kinetochore reproduction theory are: (i) breakage of dicentric chromosomes between centromere pairs forms acrocentric derivatives, (ii) de novo capping of newly synthesized acrocentric ends with telomeric DNA stabilizes these derivatives, and (iii) mitotic checkpoints regulate chromosomal disjunction to generate fissioned karyotypes. Subsequent chromosomal rearrangement, especially pericentric inversion, increases the probability of genetic isolation amongst incipient sympatric species polytypic for fission-generated acrocentric autosomes. This mechanism obviates the requirement for numerous independent Robertsonian rearrangements and neatly accounts for mammalian karyotype evolution as exemplified in analyses of Carnivora, Artiodactyla, and Primates.

Constitutional genetic markers of aging

Constitutional genetic markers of aging can be defined as members of that subset of genes that modulate the times of onset and/or the rates of progression of one or more of the processes of aging, or the response of the target cells, tissues and organisms to a particular process. These genetic factors are classified into: (1) those that control changes in structure and function that may be universally expressed in aging organisms or that are expressed in large taxonomic groups of organisms ("public markers") and (2) those that control changes that are species specific or that reflect polymorphisms or mutations within a species ("private markers"). Both spontaneous and experimentally induced genetic variation can identify and characterize such genetic elements. Recommendations for implementing such a program of research include (1) particularization of the aging phenotype, (2) further development of nonmammalian models amenable to genetic analysis, (3) systematic search for relevant spontaneous mutations in Mus musculus, (4) utilization of recombinant inbred, chimeric, transgenic and interspecific mice and (5) investigations of genetic concomitants of speciation.