Thirty years of artificial syntopy between Hydromantes italicus and H. ambrosii ambrosii (Amphibia, Plethodontidae)

Abstract

Thirty years ago, in 1983, an experiment of artificial syntopy put together two allopatric taxa of mainland European plethodontids: Hydromantes ambrosii ambrosii and H. italicus. An equal number of specimens of both species were released in a cave with a suitable environment but located outside the range of the genus. The aim was to test the effectiveness of the reproductive isolating mechanisms of these two moderately divergent taxa and, in the case hybridization would have occurred, to analyse the extent and mechanisms of introgressive hybridization while in progress. Previous data collected between 1996 and 1999 showed that H. italicus and H. a. ambrosii were hybridizing and that their hybrids were viable and fertile enough to produce backcrosses. The data presented here, based on allozymes and restriction enzymes on a fragment of the mitochondrial Cytochrome-b gene, showed that introgressive hybridization is still ongoing. However, the gene exchange between the two taxa is restricted since most of the specimens scored were pure H. a. ambrosii, the percentage of hybrid/recombinant specimens was quite low and a strong deficiency of heterozygote genotypes was recorded. The results presented showed that this long term experiment assisted in providing insights into the patterns and mechanisms underlying hybridization and introgression, showing the spreading of a foreign mtDNA (H. italicus) into the gene pool of another species (H. a. ambrosii) despite the fact that the nuclear genomes remain substantially differentiated.