Complementary approaches of cytogenetics and molecular biology to the taxonomy and study of speciation processes in lemurs

Natural male hybrid common shrews with a very long chromosomal multivalent at meiosis appear not to be completely sterile

Among 36 known chromosomal hybrid zones of the common shrew Sorex araneus, the Moscow-Seliger hybrid zone is of special interest because inter-racial complex heterozygotes (F1 hybrids) produce the longest meiotic configuration, consisting of 11 chromosomes with monobrachial homology (undecavalent or chain-of-eleven: CXI). Different studies suggest that such a multivalent may negatively affect meiotic progression and in general should significantly reduce fertility of hybrids. In this work, by immunocytochemical and electron microscopy methods, we investigated for the first time chromosome synapsis, recombination and meiotic silencing in pachytene spermatocytes of natural inter-racial heterozygous shrew males carrying CXI configurations. Despite some abnormalities detected in spermatocytes, such as associations of chromosomes, stretched centromeres, and the absence of recombination nodules in some arms of the multivalent, a large number of morphologically normal spermatozoa were observed. Possible low stringency of pachytene checkpoints may mean that even very long meiotic configurations do not cause complete sterility of such complex inter-racial heterozygotes.

Analysis of meiotic chromosome structure and behavior in Robertsonian heterozygotes of Ellobius tancrei (Rodentia, Cricetidae): a case of monobrachial homology

Synaptonemal complex (SC) chains were revealed in semisterile intraspecific F1 hybrids of Ellobius tancrei Blasius, 1884 (2n = 49, NF=56 and 2n=50, NF=56), heterozygous for Robertsonian (Rb) translocations. Chains were formed by Rb submetacentrics with monobrachial homology. Chromosome synapsis in spermatocytes of these hybrids was disturbed, apparently because of the problematic release of the chromosomes from the SC chains. These hybrids suffer from low fertility, and our data support the opinion that this is because a formation of Rb metacentrics with monobrachial homology within different races of the same species might be an initial event for the divergence of chromosomal forms.

Synaptonemal complex analysis of interracial hybrids between the Moscow and Neroosa chromosomal races of the common shrew Sorex araneus showing regular formation of a complex meiotic configuration (ring-of-four)

Immunocytochemical and electron microscopic analysis of synaptonemal complexes (SCs) was carried out for the first time in homozygotes and complex Robertsonian heterozygotes (hybrids) of the common shrew, Sorex araneus Linnaeus, 1758, from a newly discovered hybrid zone between the Moscow and the Neroosa chromosomal races. These races differ in four monobrachial homologous metacentrics, and closed SC tetravalent is expected to be formed in meiosis of a hybrid. Indeed, such a multivalent was found at meiotic prophase I in hybrids. Interactions between multivalent and both autosomes and/or the sex chromosomes were observed. For the first time we have used immunocytochemical techniques to analyse asynapsis in Sorex araneus and show that the multivalent pairs in an orderly fashion with complete synapsis. Despite some signs of spermatocytes arrested in the meiotic prophase I, hybrids had large number of active sperm. Thus, Moscow - Neroosa hybrid males that form a ring-of-four meiotic configuration are most likely not sterile. Our results support previous demonstrations that monobrachial homology of metacentrics of the common shrew does not lead to complete reproductive isolation between parapatric chromosomal races of the species.

Complete sequence and gene organization of the mitochondrial genome for Hubbard's sportive lemur (Lepilemur hubbardorum)

The complete mitochondrial DNA (mtDNA) genome of Hubbard's or Zombitse sportive lemur (Lepilemur hubbardorum) was generated by polymerase chain reaction (PCR) amplification, primer-walking sequencing and fragment cloning. Comparative analyses of Hubbard's sportive lemur were conducted with available complete mitochondrial genome sequences from eight other lemur species. The mitochondrial genome of Hubbard's sportive lemur is 16,854 base pairs (bp) and contains 13 protein-coding genes, 22 transfer RNA (tRNA) genes, two ribosomal RNA (rRNA) genes and one control region. Three rare start codons were found, in which GTG is the start codon for the ATPase 6 subunit gene (ATP), ATC for the NADH dehydrogenase (ND) 2 subunit gene, and ATT for the ND5 subunit gene. In the control region, sequence analysis found one repetitive unit between conserved sequence blocks (CSB)-1 and CSB-2 for L. hubbardorum. Comparative analysis of eight other lemur species showed different repetitive units between and outside of these two blocks. According to the phylogenetic analysis of the 12 heavy-strand encoded protein-coding genes, all nine lemur species representative of four lemuriformes families were monophyletic. This template and the newly designed primers described in this study will allow scientists to generate comparative sequences for all sportive lemurs to validate phylogenetic discrepancies in the genus Lepilemur and to evaluate evolutionary and biogeographic models.

Comparing chromosomal and mitochondrial phylogenies of sportive lemurs (Genus Lepilemur, Primates)

In recent years several new sportive lemur species (genus Lepilemur) have been described. In contrast to other lemur taxa, the genus shows comparatively high chromosomal variability, which, in addition to molecular data, can be used to infer phylogenetic relationships within the genus. By comparing R-banding patterns and fluorescence in-situ hybridization data, we detected chromosomal rearrangements that occurred during speciation within the genus. The analysis of these data with cladistic methods resulted in a dichotomic phylogenetic tree comparable to that obtained from mitochondrial sequence data. However, a phase of reticulation can not be excluded from the evolution of Lepilemur. Although some incongruences were detected, both phylogenies show similar patterns concerning relationships of the basal and terminal splits. We therefore hypothesize that both, chromosome rearrangements and molecular mutations, alone or in combination, contributed to the speciation process in sportive lemurs.

Unprecedented chromosomal diversity and behaviour modify linkage patterns and speciation potential: structural heterozygosity in an Australian spider

The Huntsman spider Delena cancerides shows an extraordinary level of chromosomal diversity and meiotic complexity. Some populations form normal bivalents at male meiosis, but 14 populations form chains of chromosomes. Six of these populations form two chains, and so show segregation behaviour which is beyond our current understanding of meiotic processes. Chromosomal variation of this sort is rarely tolerated in other species, because the segregation of long chromosome chains frequently results in gametes with too many or too few chromosomes. The resulting reproductive failure may form the basis for reproductive isolation in many species, and so the mechanisms that allow D. cancerides to segregate long chromosome chains have allowed this species to maintain cohesion despite extensive chromosomal variation over its range. The effect these chromosome chains have on the population genetics of the species is discussed, and a model for the evolution of the system is proposed.

Biodiversity, phylogeography, biogeography and conservation: lemurs as an example

The lemurs of Madagascar represent a spectacular example of adaptive radiation among primates. Given the special setting under which they evolved (i.e. long isolation, geographical location, geological relief), they provide excellent models for study in many realms, and at different levels and scales, including diversity. At the same time, they occur in a 'hottest hot spot' region for biodiversity conservation. Although there is no single definition of biodiversity, the most commonly used units to measure biodiversity are species-species richness, species abundance and, for conservation purposes in particular, species endemism. However, what a species actually is or how, precisely, it should be defined are unresolved issues. Many species concepts have been proposed and several have been used in primatology in recent years. Nowadays, one of the more common approaches to measuring diversity, and eventually inferring species status, is to look at genetic diversity as reflected by mitochondrial DNA differences. Not enough attention has been paid, however, to the different levels at which genetic differences may occur. Lemurs provide instructive examples to highlight the questions involved in species recognition and definition. Using lemurs as examples, I will highlight the strengths and limitations of some analytical tools, including phylogeography and cladistic biogeography and, I will, in particular, emphasize the questions arising at the interface of scientific and conservation perceptions, both of which influence decisions in the field of biodiversity preservation.