Variation In Number Of Loci Research Articles

Physical mapping of 5S and 18S-5.8S-26S RNA gene families in polyploid series of Cenchrus ciliaris Linnaeus, 1771 (Poaceae)

The Buffelgrass (Cenchrus ciliaris L., Poaceae) is one of the most important pasturage grasses due to its high productivity and good forage qualities. This species possess a high adaptability to bioclimatic constraints of arid zones and may be used for the restoration of degraded arid ecosystems. Tunisian populations present three ploidy levels (4x, 5x and 6x) with a basic chromosome number x=9. This study reported for the first time the distribution of the ribosomal genes (rRNA) for pentaploid and hexaploid cytotypes of Cenchrus ciliaris. Molecular cytogenetic study using double fluorescence in situ hybridization has shown that the two rDNA families, 5S and 18S-5.8S-26S (18S), displayed intraspecific variation in number of loci among different ploidy levels. Each ploidy level was characterized by specific number of both 5S and 18S rDNA loci (two loci in tetraploid, five in pentaploid and six in hexaploid level). For three studied cytotypes (4x, 5x and 6x) all 5S rDNA loci were localized on the subcentromeric region of chromosomes, while 18S loci were situated on the telomeric region of short chromosome arms. Data of the FISH experiments show proportional increase of ribosomal loci number during polyploidization processes.

Mhc class I genes of swordtail fishes, Xiphophorus: variation in the number of loci and existence of ancient gene families.

Swordtail fishes and platies in the genus Xiphophorus (order Cyprinodontiformes, Teleostei) encompass 22 closely related species which are the products of a recent adaptive radiation in the streams of Central America. To investigate the evolution of the major histocompatibility complex (Mhc) genes in the period immediately following speciation, the class I genes from 20 of the 22 species were cloned and characterized by sequencing. The analysis revealed the existence of multiple loci (at least seven in some individuals) whose numbers vary among the different species and probably also among individuals of the same species. The variation does not seem to bear any relationship to the taxonomy of the genus. Genes at the different loci are distinguished by their intron sequences and by the presence of characteristic motifs in exons 2 and 3. The variation in copy number of loci may have been effected in part by unequal crossing over occurring between introns of misaligned closely related genes. The sequences of the genes fall into two groups, A and B, which represent ancient lineages. The groups define two families of loci, which diverged from each other an estimated 85 million years ago, before the separation of the Acanthopterygii from the Paracanthopterygii of the advanced bony fishes. Evolution of the genes within each family can be explained by the birth-and-death process driven by gene duplications and mutational differentiation.