Analysis of rDNA ITS1 indels in Caulerpa taxifolia (Chlorophyta) supports a derived, incipient species status for the invasive strain

We analysed insertion-deletion patterns in 159 published sequences of ITSI for Caulerpa taxifolia (Vahl) C. Agardh collected from 55 localities throughout the species' range. Five indelotypes (I) were identified that represented a sequential loss of insertions from the ancestral type (I-3) to the most derived type (I-0). The I-3 consists of the complete ITSI sequence, which is also characteristic of three outgroup species. In contrast, the I-0 has lost three inserts from the complete sequence and is associated with the invasive forms found in the Mediterranean, California and southeastern Australia. The I-2 was found in samples from the Red Sea and Jakarta, whereas the I-1 was associated with samples from Australia and New Caledonia. When mapped onto location and habitat, these ordered character-states reveal a widespread distribution of I-3 and (probably) I-2, which are associated with offshore coral reefs and clear oligotrophic waters. The I-1 and I-0 comprise a paraphyletic assemblage of the more derived types harbouring two or three deletions and occurring together along mainland Australian coasts in more turbid environments. The presence of I-0, I-1a and I-1b along the Australian coast indicates that the ecological transition that gave rise to the coastal ecotype has been present at least since the time of the mutation between I-2 and I-1. These types of fixed differences confirm that C. taxifolia consists of at least two incipient species-the coastal form being an offshoot derived from the clear-water ecotypes. The finding of indelotype I-1a in an isolate from Sousse (Tunisia) confirms a second Mediterranean introduction and highlights the urgency for further research in the evolutionary diversification of one of the most intriguing and troublesome seaweeds.