PHYLOGENETIC RELATIONSHIPS OFCAULERPA(CHLOROPHYTA) BASED ON COMPARATIVE CHLOROPLAST ULTRASTRUCTURE1,2

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Abstract
SUMMARY: The ultrastructure of chloroplasts from 28 of the 73 species ofCaulerpaLamouroux (Chlorophyta, Caulerpales) has been studied to aid in interpreting phylogenetic relationships among the 12 recognized sections. Variations of systematic value include pyrenoid occurrence and fine structure, thylakoid architecture and amount of photosynthate storage. Comparisons of field and culture specimens indicate these characters are consistent. Chloroplast thylakoids are grouped into bands, with the distribution of bands differing among species. In the most common arrangement, bands are evenly distributed throughout the chloroplast. A few species show lateral displacement of bands whereas others have a majority of bands arranged at one end of the chloroplast. Starch is stored cither as one or two large grains (> 1 μm diam.) or numerous small grains (< 0.5 μm diam.). Electron‐transparent regions are common in other species in which chloroplasts rarely store starch. Simple, embedded pyrenoids are present in several species of section Sedoideae. An opaque region occurs in chloroplasts ofC. elongatawhich may represent an intermediate stage in the evolutionary loss of the pyrenoid. It is suggested that the chloroplast ofCaulerpaevolved, from a large, complex, pyrenoid‐containing organelle housing both photosynthetic and amylogenic functions, to a small, structurally simpler one, specialized for photosynthesis alone. A phylogeny of the 12 sections ofCaulerpais constructed, based on chloroplast evolution which agrees with an earlier morphology‐based hypothesis on the origin and evolution ofCaulerpa.