International study on Artemia. LXII. Genomic relationships between Artemia franciscana and A-persimilis, inferred from chromocentre numbers

Chromocentres, i.e. heavily stainable heterochromatic areas with highly rep etitive DNA (a 130-bp repeat in the order of 6 x 10(5) Copies per haploid g enome) observed in the resting nucleus, are a reliable taxonomic trait and a good marker for speciation in Artemia. This chromosome marker was evaluat ed in populations of two New World sibling species: A. franciscana, from No rth, Central and South America, and A. persimilis from Argentina. Artemia p ersimilis showed the characteristically low average chromocentre numbers ( < 3) of the ancestral Mediterranean species, whilst higher numbers (> 5) we re seen in A. franciscana. The increase in chromocentre numbers from A. per similis to A. franciscana occurred in a rather steady, continuous geographi cal pattern with an overlapping zone between both species. A north-south st eady latitudinal decline of chomocentres was observed towards the equator i n A. franciscana populations from the northern hemisphere, whilst the trait increased from this point towards southern latitudes with a population in Chilean Patagonia (below 52 degreesS) exhibiting the highest chromocentre f requency. Since chromocentres are positively correlated with repetitive DNA content. differentiation between the species through the amplification of heterochromatin or an increased DNA content in A. franciscana (although thi s species has a lower diploid chromosome number), might nevertheless reflec t chromosomal reorganizations between the species. Chromocentres, together with other traits, reveal a probable hybrid zone between the two species in Chilean Patagonia.