The Single, Ancient Origin of Chromist Plastids

Abstract

Algae include a diverse array of photosynthetic eukaryotes excluding land plants. Explaining the origin of algal plastids continues to be a major challenge in evolutionary biology. Current knowledge suggests that plastid primary endosymbiosis, in which a singlecelled protist engulfs and ''enslaves'' a cyanobacterium, likely occurred once and resulted in the primordial alga. This eukaryote then gave rise through vertical evolution to the red, green, and glaucophyte algae. However, some modern algal lineages have a more complicated evolutionary history involving a secondary endosymbiotic event, in which a protist engulfed an existing eukaryotic alga (rather than a cyanobacterium), which was then reduced to a secondary plastid. Secondary endosymbiosis explains the majority of algal biodiversity, yet the number and timing of these events is unresolved. Here we analyzed a five-gene plastid data set to show that a taxonomically diverse group of chlorophyll c 2-containing protists comprising cryptophyte, haptophyte, and stramenopiles algae (Chromista) share a common plastid that most likely arose from a single, ancient (Ϸ1,260 million years ago) secondary endosymbiosis involving a red alga. This finding is consistent with Chromista monophyly and implicates secondary endosymbiosis as an important force in generating eukaryotic biodiversity.cryptophyte ͉ haptophyte ͉ plastid evolution ͉ secondary endosymbiosis ͉ stramenopiles

Adaptive signals in algal Rubisco reveal a history of ancient atmospheric carbon dioxide

“…Bayesian tree reconstruction and dating were conducted using BEAST software [23]. The calibration constraints used for each tree are summarized in electronic supplementary material, table S3 [13][14][15][24][25][26]. Phylogeny reconstruction assumed the general time-reversible substitution model [27] with gamma-distributed rate heterogeneity.…”

“…Chromalveolata includes the Chromista (Haptophyta, Stramenopiles and Cryptophyta) [11] and the Alveolata. However, as there is debate on the plastid monophyly of the Chromalveolata (for review, see Keeling [12]), we focused only on the Chromista which have a wellsupported single secondary endosymbiotic origin of their plastids [13]. Furthermore, many representatives of the Chromista algae, such as diatoms and Haptophyta, dominate the modern ocean and display an extensive fossil history [14,15], making it possible to date the periods of adaptive evolution in Rubisco.…”

Origin and Distribution of Calvin Cycle Fructose and Sedoheptulose Bisphosphatases in Plantae and Complex Algae: A Single Secondary Origin of Complex Red Plastids and Subsequent Propagation via Tertiary Endosymbioses

“…In contrast, a common ancestry of stramenopiles (oomycetes, heterokont algae), haptophytes, cryptophytes, and alveolates (ciliates, apicomplexa, dinoflagellates) via a single secondary endosymbiosis with a rhodophyte was recently proposed and is known as the "chromalveolate hypothesis" (Cavalier-Smith 1999). This hypothesis is supported by plastid phylogenies in which the "chromalveolate" lineages are monophyletic and nested within red algae (Bachvaroff et al 2005;Yoon et al 2002). At the nuclear level, three genes of the Calvin cycle, i.e.…”

The collapse of the two-kingdom system, the rise of protistology and the founding of the International Society for Evolutionary Protistology (ISEP)

“…Wellknown exceptions include the size reduction of the cristae in low-oxygen conditions, as in some trypanosomes. The position of cryptomonads continues to be troublesome, with some molecular evidence indicating the placement of cryptomonads (flat cristae) with the chromalveolates (tubular) (Fast et al, 2001;Yoon et al, 2002), although the use of plastid characters to place the 'host' phylogenetic position can be questioned. The position of Dictyostelium (tubular cristae) closer to animals (flat) than plants (also flat) (Bapteste et al, 2002) contradicts the monophyly of flat cristae.…”