Genetic variation in gorillas

This review summarizes what is currently known concerning genetic variation in gorillas, on both inter- and intraspecific levels. Compared to the human species, gorillas, along with the other great apes, possess greater genetic variation as a consequence of a demographic history of rather constant population size. Data and hence conclusions from analysis of mitochondrial DNA (mtDNA), the usual means of describing intraspecific patterns of genetic diversity, are limited at this time. An important task for future studies is to determine the degree of confidence with which gorilla mtDNA can be analyzed, in view of the risk that one will inadvertently analyze artifactual rather than genuine sequences. The limited information available from sequences of nuclear genomic segments does not distinguish western from eastern gorillas, and, in comparison with results from the two chimpanzee species, suggests a relatively recent common ancestry for all gorillas. In the near future, the greatest insights are likely to come from studies aimed at genetic characterization of all individual members of social groups. Such studies, addressing topics such as behavior of individuals with kin and non-kin, and the actual success of male reproductive strategies, will provide a link between behavioral and genetic studies of gorillas.     

Rapid sequence turnover at an intergenic locus in Drosophila

Closely related species of Drosophila tend to have similar genome sizes. The strong imbalance in favor of small deletions relative to insertions implies that the unconstrained DNA in Drosophila is unlikely to be passively inherited from even closely related ancestors, and yet most DNA in Drosophila genomes is intergenic and potentially unconstrained. In an attempt to investigate the maintenance of this intergenic DNA, we studied the evolution of an intergenic locus on the fourth chromosome of the Drosophila melanogaster genome. This 1.2-kb locus is marked by two distinct, large insertion events: a nuclear transposition of a mitochondrial sequence and a transposition of a nonautonomous DNA transposon DNAREP1_DM. Because we could trace the evolutionary histories of these sequences, we were able to reconstruct the length evolution of this region in some detail. We sequenced this locus in all four species of the D. melanogaster species complex: D. melanogaster, D. simulans, D. sechellia, and D. mauritiana. Although this locus is similar in size in these four species, less than 10% of the sequence from the most recent common ancestor remains in D. melanogaster and all of its sister species. This region appears to have increased in size through several distinct insertions in the ancestor of the D. melanogaster species complex and has been shrinking since the split of these lineages. In addition, we found no evidence suggesting that the size of this locus has been maintained over evolutionary time; these results are consistent with the model of a dynamic equilibrium between persistent DNA loss through small deletions and more sporadic DNA gain through less frequent but longer insertions. The apparent stability of genome size in Drosophila may belie very rapid sequence turnover at intergenic loci.     

Assessing the effects of primer specificity on eliminating numt coamplification in DNA barcoding: a case study from Orthoptera (Arthropoda: Insecta)

DNA barcoding is a diagnostic method of species identification based on sequencing a short mitochondrial DNA fragment of cytochrome oxidase I (COI), but its ability to correctly diagnose species is limited by the presence of nuclear mitochondrial pseudogenes (numts). Numts can be coamplified with the mitochondrial orthologue when using universal primers, which can lead to incorrect species identification and an overestimation of the number of species. Some researchers have proposed that using more specific primers may help eliminate numt coamplification, but the efficacy of this method has not been thoroughly tested. In this study, we investigate the taxonomic distribution of numts in 11 lineages within the insect order Orthoptera, by analysing cloned COI sequences and further test the effects of primer specificity on eliminating numt coamplification in four lineages. We find that numts are coamplified in all 11 taxa using universal (barcoding) primers, which suggests that numts may be widespread in other taxonomic groups as well. Increased primer specificity is only effective at reducing numt coamplification in some species tested, and only eliminates it in one species tested. Furthermore, we find that a number of numts do not have stop codons or indels, making it difficult to distinguish them from mitochondrial orthologues, thus putting the efficacy of barcoding quality control measures under question. Our findings suggest that numt coamplification is a serious problem for DNA barcoding and more quality control measures should be implemented to identify and eliminate numts prior to using mitochondrial barcodes for species diagnoses.     

Barcoding Queensland Fruit Flies (Bactrocera tryoni): impediments and improvements

Identification of adult fruit flies primarily involves microscopic examination of diagnostic morphological characters, while immature stages, such as larvae, can be more problematic. One of the Australia’s most serious horticultural pests, the Queensland Fruit Fly (Bactrocera tryoni: Tephritidae), is of particular biosecurity/quarantine concern as the immature life stages occur within food produce and can be difficult to identify using morphological characteristics. DNA barcoding of the mitochondrial Cytochrome Oxidase I (COI) gene could be employed to increase the accuracy of fruit fly species identifications. In our study, we tested the utility of standard DNA barcoding techniques and found them to be problematic for Queensland Fruit Flies, which (i) possess a nuclear copy (a numt pseudogene) of the barcoding region of COI that can be co‐amplified; and (ii) as in previous COI phylogenetic analyses closely related B. tryoni complex species appear polyphyletic. We found that the presence of a large deletion in the numt copy of COI allowed an alternative primer to be designed to only amplify the mitochondrial COI locus in tephritid fruit flies. Comparisons of alternative commonly utilized mitochondrial genes, Cytochrome Oxidase II and Cytochrome b, revealed a similar level of variation to COI; however, COI is the most informative for DNA barcoding, given the large number of sequences from other tephritid fruit fly species available for comparison. Adopting DNA barcoding for the identification of problematic fly specimens provides a powerful tool to distinguish serious quarantine fruit fly pests (Tephritidae) from endemic fly species of lesser concern.     

Can genomics clarify the origins of Boreioglycaspis melaleucae in California,USA?

The Australian psyllid Boreioglycaspis melaleucae is a biological control agent of Melaleuca quinquenervia in Florida (USA) but was observed attacking M. quinquenervia trees in southern California (USA). Genotyping revealed the California population matched three of eight Australian haplotypes and all three Florida haplotypes. It remains unclear if the California psyllid population arrived directly from Australia or via Florida.     

Forty Million Years of Independent Evolution: A Mitochondrial Gene and Its Corresponding Nuclear Pseudogene in Primates

Sequences from nuclear mitochondrial pseudogenes (numts) that originated by transfer of genetic information from mitochondria to the nucleus offer a unique opportunity to compare different regimes of molecular evolution. Analyzing a 1621-nt-long numt of the rRNA specifying mitochondrial DNA residing on human chromosome 3 and its corresponding mitochondrial gene in 18 anthropoid primates, we were able to retrace about 40 MY of primate rDNA evolutionary history. The results illustrate strengths and weaknesses of mtDNA data sets in reconstructing and dating the phylogenetic history of primates. We were able to show the following. In contrast to numt-DNA, (1) the nucleotide composition of mtDNA changed dramatically in the different primate lineages. This is assumed to lead to significant misinterpretations of the mitochondrial evolutionary history. (2) Due to the nucleotide compositional plasticity of primate mtDNA, the phylogenetic reconstruction combining mitochondrial and nuclear sequences is unlikely to yield reliable information for either tree topologies or branch lengths. This is because a major part of the underlying sequence evolution model — the nucleotide composition — is undergoing dramatic change in different mitochondrial lineages. We propose that this problem is also expressed in the occasional unexpected long branches leading to the “common ancestor” of orthologous numt sequences of different primate taxa. (3) The heterogeneous and lineage-specific evolution of mitochondrial sequences in primates renders molecular dating based on primate mtDNA problematic, whereas the numt sequences provide a much more reliable base for dating.[Reviewing Editor: Dr. Rafael Zardoya]     

Molecular phylogeny of African hinge‐back tortoises (Kinixys): implications for phylogeography and taxonomy (Testudines: Testudinidae)

We examine the phylogeography, phylogeny and taxonomy of hinge‐back tortoises using a comprehensive sampling of all currently recognized Kinixys species and subspecies and sequence data of three mitochondrial DNA fragments (2273 bp: 12S rRNA, ND4 + adjacent DNA coding for tRNAs, cytb) and three nuclear loci (2569 bp: C‐mos, ODC, R35). Combined and individual analyses of the two data sets using Bayesian and Maximum Likelihood methods suggest that the savannah species of Kinixys are paraphyletic with respect to the rainforest species K. homeana and K. erosa, and that the rainforest species may be derived from a savannah‐living ancestor. The previously recognized savannah species K. belliana was a conglomerate of three deeply divergent clades that we treat here as distinct species. We restrict the name K. belliana (Gray, 1830) to hinge‐back tortoises ranging from Angola to Burundi, while five‐clawed hinge‐back tortoises from the northernmost part of the formerly recognized range of K. belliana, together with four‐clawed tortoises from West Africa, are assigned to the species K. nogueyi (Lataste, 1886). These two species are allied to K. spekii, whereas Southeast African and Malagasy hinge‐back tortoises formerly lumped together with K. belliana represent the distinct species K. zombensis Hewitt, 1931, which is sister to K. lobatsiana. The latter two species together constitute the sister group of the rainforest species K. homeana and K. erosa. Mitochondrial data suggest that K. natalensis has a basal phylogenetic position in a clade embracing K. belliana sensu stricto, K. nogueyi and K. spekii, while nuclear data and the two data sets combined favour a sister group relationship of K. natalensis to all other hinge‐back tortoises. Phylogeographic structure is present in all wide‐ranging species and correlates in K. homeana and K. erosa with the Dahomey Gap and former rainforest refugia. The Malagasy population of K. zombensis is weakly differentiated from its South African conspecifics and further sampling is needed to determine whether there is support for the subspecific distinctness of Malagasy tortoises.     

Plastid DNA in the nucleus: New genes for old

Nuclear genomes of eukaryotes are bombarded by a continuous deluge of organellar DNA which contributes significantly to eukaryote evolution. Here, we present a new PCR-based method that allows the specific amplification of nuclear integrants of organellar DNA (norgs) by exploiting recent deletions present in organellar genome sequences. We have used this method to amplify nuclear integrants of plastid DNA (nupts) from the nuclear genomes of several nicotiana species and to study the evolutionary forces acting upon these sequences. The role of nupts in endosymbiotic evolution and the different genetic factors influencing the time available for a chloroplastic gene to be functionally relocated in the nucleus are discussed.     

Molecular analyses of mitochondrial pseudogenes within the nuclear genome of arvicoline rodents

Nuclear sequences of mitochondrial origin (numts) are common among animals and plants. The mechanism(s) by which numts transfer from the mitochondrion to the nucleus is uncertain, but their insertions may be mediated in part by chromosomal repair mechanisms. If so, then lineages where chromosomal rearrangements are common should be good models for the study of numt evolution. Arvicoline rodents are known for their karyotypic plasticity and numt pseudogenes have been discovered in this group. Here, we characterize a 4 kb numt pseudogene in the arvicoline vole Microtus rossiaemeridionalis. This sequence is among the largest numts described for a mammal lacking a completely sequenced genome. It encompasses three protein-coding and six tRNA pseudogenes that span ∼25% of the entire mammalian mitochondrial genome. It is bordered by a dinucleotide microsatellite repeat and contains four transposable elements within its sequence and flanking regions. To determine the phylogenetic distribution of this numt among the arvicolines, we characterized one of the mitochondrial pseudogenes (cytochrome b) in 21 additional arvicoline species. Average rates of nucleotide substitution in this arvicoline pseudogene are estimated as 2.3 × 10⁻⁸ substitutions/per site/per year. Furthermore, we performed comparative analyses among all species to estimate the age of this mitochondrial transfer at nearly 4 MYA, predating the origin of most arvicolines. All sequences generated in this study have been deposited within the GenBank database.