Common origin of the satellite DNAs of the Hawaiian spiders of the genus Tetragnatha: evolutionary constraints on the length and nucleotide composition of the repeats.

The present study characterizes the satellite DNA sequences of three endemic Hawaiian spiders, Tetragnatha acuta, Tetragnatha hawaiensis, and Tetragnatha quasimodo, to test the degree of conservation of these sequences within a closely related group of arthropods. The length and nucleotide composition of repeats of the three species is very similar both at intra- and inter-specific level. However, their nucleotide sequence is very divergent at the inter-specific level although conserve substantial similarity in some stretches. These results suggest a common origin of the Tetragnatha satellite DNAs and evolutionary constraint in the length and the nucleotide composition of these repeats at the inter-specific level but not in their nucleotide sequences. At the intra-specific level, the three species show a different degree of sequence identity between repeats as a result of specific historical processes. Tetragnatha hawaiensis shows a strong homogenization of the monomeric sequences. Tetragnatha quasimodo also shows strong homogenization but the actual repeats are higher-order repeats (dimers linking two divergent subfamilies of monomers). On the other hand, the existence of the three subfamilies of repeats in T. acuta, showing divergent sequence identity, both within and between subfamilies, suggests low homogenization of the repeats. Finally, evidence of gene conversion or unequal sister chromatid exchange events in T. quasimodo and T. acuta suggests that recombination is involved in the spreading and homogenization of stDNA sequences.