Actin evolution in the Volvocales

Fig. 1 from Kato-Minoura et al. 2015

Fig. 1 from Kato-Minoura et al. 2015: Genomic structure of volvocine actin and NAP genes. For comparison, previously identified sequences are also shown. Filled boxes, putative coding exons; open boxes, putative 5′ and 3′ untranslated regions. Intervening sequences are shown by solid lines. Intron positions are indicated by codon and phase numbers with reference to the three alpha-actins of vertebrates (377 amino acids) (Weber and Kabsch 1994). The conserved intron positions are linked with dotted lines. ATG, translation start codon; TAA or TGA, stop codon.

A new article in Plant Systematics and Evolution  reports on an ‘unconventional’ actin gene in the Volvocales. Called ‘novel actin-like protein’ (NAP), the gene is present, along with a traditional actin gene, in Chlamydomonas reinhardtiiC. moewusii, and Volvox carteri. A team of researches from Chuo University and the University of Tokyo, led by Takako Kato-Minoura, sequenced NAP from several other volvocine species in an effort to understand its origin and function.

NAP can replace conventional actin for some functions, but it fails to do so for others, presumably because it has a lower ability to polymerize. This is consistent with the ‘duplication and subfunctionalization‘ model of gene origins (cdesign proponentsists love to pretend that evolutionary theory has no explanation for gene origins). By examining whole genome sequences of other Chlorophyte algae and plants, Dr. Kato-Minoura’s team found that NAP appears to be restricted to the Volvocales. An inferred gene tree of NAP sequences doesn’t contradict the (more or less) consensus view of volvocine phylogeny, but because of low support values, it’s hard to conclude much from this. Because NAP is present in both C. reinhardtii and C. moewusii, the authors conclude that it must have been present in their common ancestor, which would suggest that its origin precedes by a good bit the origin of multicellularity in the volvocine algae.

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