Molecular Biology and Evolution, Vol 16, 1308-1313, Copyright © 1999 by Society for Molecular Biology and Evolution
PJ Keeling, JA Deane, C Hink-Schauer, SE Douglas, UG Maier and GI McFadden
Cryptomonads have acquired photosynthesis through secondary endosymbiosis:
they have engulfed and retained a photosynthetic eukaryote. The remnants of
this autotrophic symbiont are severely reduced, but a small volume of
cytoplasm surrounding the plastid persists, along with a residual nucleus
(the nucleomorph) that encodes only a few hundred genes. We characterized
tubulin genes from the cryptomonad Guillardia theta. Despite the apparent
absence of microtubules in the endosymbiont, we recovered genes encoding
alpha-, beta-, and gamma-tubulins from the nucleomorph genome of G. theta.
The presence of tubulin genes in the nucleomorph indicates that some
component of the cytoskeleton is still present in the cryptomonad symbiont
despite the fact that very little cytoplasm remains, no mitosis is known in
the nucleomorph, and microtubules have never been observed anywhere in the
symbiont. Phylogenetic analyses with nucleomorph alpha- and beta-tubulins
support the origin of the cryptomonad nucleomorph from a red alga. We also
characterized alpha and beta-tubulins from the host nucleus of G. theta and
compared these with tubulins we isolated from two flagellates, Goniomonas
truncata and Cyanophora paradoxa, previously proposed to be related to the
cryptomonad host. Phylogenetic analyses support a relationship between the
cryptomonad host and Goniomonas but do not support any relationship between
cryptomonads and Cyanophora.
ORIGINAL ARTICLE
The secondary endosymbiont of the cryptomonad Guillardia theta contains alpha-, beta-, and gamma-tubulin genes
Plant Cell Biology Research Centre, Botany School, University of Melbourne, Australia.
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