Two Functional Copies of the DGCR6 Gene Are Present on Human Chromosome 22q11 Due to a Duplication of an Ancestral Locus

The DGCR6 (DiGeorge critical region) gene encodes a putative protein with sequence similarity to gonadal(gdl), a Drosophila melanogaster gene of unknown function. We mapped the DGCR6 gene to chromosome 22q11 within a low copy repeat, termed sc11.1a, and identified a second copy of the gene, DGCR6L, within the duplicate locus, termed sc11.1b. Both sc11.1 repeats are deleted in most persons with velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS), and they map immediately adjacent and internal to the low copy repeats, termed LCR22, that mediate the deletions associated with VCFS/DGS. We sequenced genomic clones from both loci and determined that the putative initiator methionine is located further upstream than originally described, but in a position similar to the mouse and chicken orthologs.DGCR6L encodes a highly homologous, functional copy ofDGCR6, with some base changes rendering amino acid differences. Expression studies of the two genes indicate that both genes are widely expressed in fetal and adult tissues. Evolutionary studies using FISH mapping in several different species of ape combined with sequence analysis of DGCR6 in a number of different primate species indicate that the duplication is at least 12 million years old and may date back to before the divergence of Catarrhines from Platyrrhines, 35 mya. These data suggest that there has been selective evolutionary pressure toward the functional maintenance of both paralogs. Interestingly, a full-length HERV-K provirus integrated into the sc11.1a locus after the divergence of chimpanzees and humans.

How often do duplicated genes evolve new functions?

A recently duplicated gene can either fix a null allele (becoming a pseudogene) or fix an (advantageous) allele giving a slightly different function, starting it on the road to evolving a new function. Here we examine the relative probabilities of these two events under a simple model. Null alleles are assumed to be neutral; linkage effects are ignored, as are unequal crossing over and gene conversion. These assumptions likely make our results underestimates for the probability that an advantageous allele is fixed first. When new advantageous mutations are additive with selection coefficient s and the ratio of advantageous to null mutations is rho, the probability an advantageous allele is fixed first is ([1 - e-s]/[rho S] + 1)-1, where S = 4Nes with Ne the effective population size. The probability that a duplicate locus becomes a pseudogene, as opposed to evolving a new gene function, is high unless rhoS > 1. However, even if advantageous mutations are very rare relative to null mutations, for sufficiently large populations rhoS > 1 and new gene function, rather than pseudogene formation, is the expected fate of most duplicated genes.

Evolutionary Patterns in Some Putative Australian Species in the Ant Genus Rhytidoponera

Genic and morphological variation were compared for 17 putative Rhytidoponera species and a species of the related genus Heteroponera, by use of an allozyme data set and one based on morphometric, surface sculpture, and pilosity characters. Each data set was considered in three versions: the raw data, principal factor scores normalized to the appropriate eigen vectors, and these scores range-coded. The agreement between these data sets, and similar sets derived from published vertebrate studies, was gauged by means of correlation coefficients between distance matrices based on them, calculated by a jack-knife procedure. In all cases, the raw allozyme data sets gave the highest correlation with the morphological sets, but none of the treatments of the morphological data was clearly superior in this regard to the others. For the ant data, congruence between the two types of data was also examined by comparing the branching orders of dendrograms (Wagner and REML), by a new test employing distributions based on the differences between randomly generated branching orders and a reference dendrogram. According to this test, the morphological dendrogram based on range-coded principal-factor scores was significantly more similar to that derived from the raw allozyme data than were those based on the other two treatments of the data. Differences in chromosome number do not correlate well with genic and morphological ones, which indicates that the speed of karyotype change in this genus has been highly variable. Some OTUS showed duplicate-locus expression for IDH, and clustering in the allozyme- based dendrograms occurred on the basis of IDH duplicate-locus expression pattern. The two 'victoriae' populations studied cluster closely with the metallica group on the morphology-based dendrograms, in agreement with conventional views that 'victoriae' is very close to 'metallica', but diverge markedly when allozymes are considered, which indicates that the morphological resemblance is probably due to convergence. The large genetic distance between these 'victoriae' populations indicates the likely presence of sibling species. R. 'tasrnaniensis' populations, in contrast, cluster strongly with 'metallica' in both morphology- and allozyme-based dendrograms. The marked divergence of scabra from other large species in the allozyme- based dendrograms indicates that its large body size has been derived independently.