The heterochromatin of grasshoppers from the Caledia captiva species complex. I. Sequence evolution and conservation in a highly repeated DNA family

High-throughput analysis of the satellitome illuminates satellite DNA evolution

Satellite DNA (satDNA) is a major component yet the great unknown of eukaryote genomes and clearly underrepresented in genome sequencing projects. Here we show the high-throughput analysis of satellite DNA content in the migratory locust by means of the bioinformatic analysis of Illumina reads with the RepeatExplorer and RepeatMasker programs. This unveiled 62 satDNA families and we propose the term "satellitome" for the whole collection of different satDNA families in a genome. The finding that satDNAs were present in many contigs of the migratory locust draft genome indicates that they show many genomic locations invisible by fluorescent in situ hybridization (FISH). The cytological pattern of five satellites showing common descent (belonging to the SF3 superfamily) suggests that non-clustered satDNAs can become into clustered through local amplification at any of the many genomic loci resulting from previous dissemination of short satDNA arrays. The fact that all kinds of satDNA (micro- mini- and satellites) can show the non-clustered and clustered states suggests that all these elements are mostly similar, except for repeat length. Finally, the presence of VNTRs in bacteria, showing similar properties to non-clustered satDNAs in eukaryotes, suggests that this kind of tandem repeats show common properties in all living beings.

Ribosomal RNA-encoding DNA introgression across a narrow hybrid zone between two subspecies of grasshopper

A ribosomal RNA-encoding DNA (rDNA) cloned sequence, consisting of a 0.8-kilobase fragment from the 26S/nontranscribed spacer region, was used to identify diagnostic restriction enzyme fragments that distinguish the Moreton and Torresian subspecies of the grasshopper Caledia captiva. These restriction fragments were then used to study patterns of rDNA variation across a narrow geographical hybrid zone between the two subspecies. The pattern of rDNA variation that emerged after the analysis of over 250 individuals clearly demonstrates the asymmetrical introgression of the Moreton ribosomal RNA genes into the Torresian subspecies. This asymmetric movement of genetic material occurs even though there exists extreme postmating F(2) and backcross inviability between the two subspecies. From our data, as well as those of previous chromosomal and allozymic studies, we are able to support the occurrence of nonrandom processes such as biased gene conversion and/or natural selection. Because the rDNA loci in the Moreton and Torresian individuals are located in different regions on chromosomes 10 and 11, it should be possible to determine the relative contributions of conversion, natural selection, and these sorts of processes to the pattern of introgression of the Moreton rDNA into the Torresian subspecies.

High heterochromatin content in somatic chromosomes of two unrelated species of Diplopoda (Myriapoda)

For the first time, a conventional analysis of C-banded karyotypes was carried out in two distantly related diplopod species; this revealed an impressive percentage of heterochromatin in both genomes. In Acanthopetalum sicanum (Order Callipodida) (2n = 12), heterochromatin constitutes about 60% of the total DNA in females and 56% in males, whereas in Enologus oxypygum (Order Julida) (2n = 22) it is about 67% in both sexes. Heterochromatin of the two species was found to be similar in base composition (AT rich) and heterochromatin distribution, indicating that it has accumulated in a species-specific manner. Sex-determining mechanisms of the XY type were detected in both A. sicanum and E. oxypygum. In A. sicanum, the Y presented the lowest heterochromatic content of all chromosomes in the karyotype, whereas the X presented the highest.