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

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Abstract
We analysed insertion-deletion patterns in 159 published sequences of ITS1 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 (I3) to the most derived type (I0). The I3 consists of the complete ITS1 sequence, which is also characteristic of three outgroup species. In contrast, the I0 has lost three inserts from the complete sequence and is associated with the invasive forms found in the Mediterranean, California and southeastern Australia. The I2 was found in samples from the Red Sea and Jakarta, whereas the I1 was associated with samples from Australia and New Caledonia. When mapped onto location and habitat, these ordered character-states reveal a widespread distribution of I3 and (probably) I2, which are associated with offshore coral reefs and clear oligotrophic waters. The I1 and I0 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 I0, I1a and I1b 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 I2 and I1. 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 I1a 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.