Pigment composition and rbcL sequence data from the silicoflagellate Dictyocha speculum:: A heterokont alga with pigments similar to some haptophytes

Monophyly of plastids in the morphologically diverse heterokont algae has rarely been questioned. However, HPLC analysis revealed that the pigment composition of the silicoflagellate Dictyocha speculum Ehrenberg is similar to that observed in a group of haptophytes (type 4 sensu Jeffrey and Wright 1994). Dictyocha speculum and type 4 haptophytes possess acylfucoxanthins (19'-butanoyloxy- and 19'-hexanoyloxyfucoxanthin) in addition to fuco-, diadino-, and diatoxanthin and chl a, c, and c(3). The pigment composition of two pedinellids (Apedinella radians [Lohmann] Campbell and Mesopedinella arctica Daugbjerg), a sister group to A speculum, deviates from D. speculum by lack of chl c(3) and acylfucoxanthins. The distinct pigment composition suggested that plastid evolution in D. speculum differs from that of other heterokont algae. This prompted determination of the plastid-encoded rbcL gene from D. speculum to gain further insight into the evolutionary history of plastids in heterokont algae and haptophytes. A phylogenetic inference based on parsimony, maximum likelihood, and LogDet transformation methods included 35 heterokonts, 19 haptophytes, 8 red algae, and I cryptomonad. Three proteobacteria possessing type I RUBISCO were used to root the tree. In phylogenetic analyses, D. speculum was closely related to Rhizochromulina sp. and pedinellids, despite the latter possessing a different pigment composition. Surprisingly, the Dictyochophyceae clustered outside the lineage of heterokont algae but not within the haptophytes. Hence, analyses deduced from rbcL sequences indicated that the plastids in heterokont algae might have a more complex evolutionary history and that the shared pigment composition in A speculum and type 4 haptophytes could be explained by convergent evolution or gene transfer. The pigment composition in D. speculum may have implications for pigment-based characterization of phytoplankton community structure in natural samples.