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Many parasitic Apicomplexa, such as Plasmodium falciparum, contain an unpigmented chloroplast remnant termed the apicoplast, which is a target for malaria treatment. However, no close relative of apicomplexans with a functional photosynthetic plastid has yet been described. Here we describe a newly cultured organism that has ultrastructural features typical for alveolates, is phylogenetically related to apicomplexans, and contains a photosynthetic plastid. The plastid is surrounded by four membranes, is pigmented by chlorophyll a, and uses the codon UGA to encode tryptophan in the psbA gene. This genetic feature has been found only in coccidian apicoplasts and various mitochondria. The UGA-Trp codon and phylogenies of plastid and nuclear ribosomal RNA genes indicate that the organism is the closest known photosynthetic relative to apicomplexan parasites and that its plastid shares an origin with the apicoplasts. The discovery of this organism provides a powerful model with which to study the evolution of parasitism in Apicomplexa. [ABSTRACT FROM AUTHOR]

Changes in the population structure of Salpa thompsoni from austral mid-summer to autumn off the Adelie Coast, Antarctica, were examined using cluster analysis based on reproductive states. S. thompsoni was the dominant macrozooplankton species through the research periods although the abundance decreased markedly in autumn. In both summer and autumn we identified three geographically coherent subpopulations, which were considered to be either a mating swarm or physical aggregation. Many aggregate forms with an empty (dead) embryo were observed, and the ratio of newborn solitary forms decreased in autumn, indicating sexual reproduction of the salps was deactivated. The significant reduction in size of mature solitaries suggested that the capacity of asexual reproduction was also reduced in autumn. In addition to water temperature decline, the phytoplankton shortage (which was partly caused by high grazing pressure of S. thompsoni during summer) might be the factor causing decline in their own population size in autumn. [ABSTRACT FROM AUTHOR]