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The Evolutionary History of Haptophytes and Cryptophytes Proc. R. Soc. B (2012) 279, 2246–2254 doi:10.1098/rspb.2011.2301 Published online 1 February 2012 The evolutionary history of haptophytes and cryptophytes: phylogenomic evidence for separate origins Fabien Burki, Noriko Okamoto, Jean-Franc¸ois Pombert and Patrick J. Keeling* Canadian Institute for Advanced Research, Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 Animportantmissingpieceinthepuzzleofhowplastidsspreadacrosstheeukaryotictreeoflifeisarobust evolutionary framework for the host lineages. Four assemblages are known to harbour plastids derived from red algae and, according to the controversial chromalveolate hypothesis, these all share a common ancestry. Phylogenomic analyses have consistently shown that stramenopiles and alveolates are closely related, but hap- tophytes and cryptophytes remain contentious; they have been proposed to branch together with several heterotrophic groups in the newly erected Hacrobia. Here, we tested this question by producing a large expressed sequence tag dataset for the katablepharid Roombia truncata, one of the last hacrobian lineages for which genome-level data are unavailable, and combined this dataset with the recently completed genome of the cryptophyte Guillardia theta to build an alignment composed of 258 genes. Our analyses strongly support haptophytes as sister to the SAR group, possibly together with telonemids and centrohelids. We also confirmed the common origin of katablepharids and cryptophytes, but these lineages were not related to other hacrobians; instead, they branch with plants. Our study resolves the evolutionary position of haptophytes, an ecologically critical component of the oceans, and proposes a new hypothesis for the origin of cryptophytes. Keywords: phylogenomics; plastid; haptophyte; cryptophyte; katablepharid; tree of life 1. INTRODUCTION haptophytes and cryptophytes, altogether forming the Eukaryotes first acquired photosynthesis through endosym- Chromalveolata supergroup [11]. This hypothesis is based biosis, where a heterotrophic cell engulfed and retained on the fact that complex events are necessary to establish a a photosynthetic prokaryote related to modern-day Cyano- plastid, so it is more parsimonious to limit the number of bacteria, ultimately integrating it to form the highly plastid origins, regardless of the number of plastid losses specialized plastid organelles we see today [1–3]. This this implies [12]. Thus far, plastid data have generally sup- crucial event in eukaryote evolution is generally seen as ported the monophyly of some or all of the chromalveolate unique: primary plastids probably evolved only once in lineages where plastids are known. Molecular evidence the common ancestor of glaucophytes, red algae and for this includes multi-gene phylogenies [13,14], shared green plants (green algae þ land plants), together making evolutionary history of several nucleus-encoded plastid- the Plantae supergroup [4](butsee[5]). A much more targeted genes [15–18], and a rare lateral gene transfer in recent case of cyanobacterium to eukaryote endosymbiosis the plastids of haptophytes and cryptophytes [19]. has been reported in the rhizarian Paulinella chromatophora The chromalveolate hypothesis also predicts that the [6], but this event appears to have had less impact on the host nuclear lineages are monophyletic; so far, however, diversification of plastids. Photosynthesis spread further to this has proven impossible to verify despite the use of other eukaryotic lineages by means of secondary endosym- substantial alignments (in the range of 30 000 amino bioses, when other eukaryotes subsequently engulfed green acids). Nuclear-based phylogenomics have consistently or red algae, and, in dinoflagellates, tertiary endosymbioses shown that stramenopiles and alveolates are closely related, [7]. On the green side, two independent cases of secondary and that they form a strongly supported group with endosymbioses are known, leading to chlorarachniophyte Rhizaria, altogether making the so-called SAR group and euglenid algae, respectively [8]. On the red side, the [20,21]. At the same time, haptophytes and cryptophytes situation is much more contentious. generally appeared together, albeit with less support and The chromalveolate hypothesis has been regarded as a only when relatively large alignments are used [21–24]. likely evolutionary framework for explaining the origin and Based on congruent plastid and nuclear data, these were distribution of red secondary plastids [9,10]. It posits that proposed to be a second chromalveolate lineage, Hacrobia a single secondary endosymbiosis with a red alga gave rise [25]. Other lineages that were not originally included in to plastids in stramenopiles (or heterokonts), alveolates, the chromalveolate hypothesis have since been suggested to be members of Hacrobia (namely telonemids, centrohelids, katablepharids, picobiliphytes, Palpitomonas and rappemo- * Author for correspondence ([email protected]). nads), but the support for these is variable, and typically Electronic supplementary material is available at http://dx.doi.org/ few data are available, from only a single representative of 10.1098/rspb.2011.2301 or via http://rspb.royalsocietypublishing.org. these lineages [22,25–32]. Received 1 November 2011 Accepted 10 January 2012 2246 This journal is q 2012 The Royal Society Haptophyte and cryptophyte evolution F. Burki et al. 2247 The large variations in the phylogenetic signal between (i.e. did not show any evidence for deep paralogy, lateral gene plastid and nuclear data have recently been formalized transfer or extensive lineage sorting). Overall, the dataset pre- in a ‘phylogenomic falsification’ of the chromalveolate sented here is composed of 258 genes. See the electronic hypothesis, which concluded that red algal plastids were supplementary material for a detailed procedure of the acquired separately in different lineages [18,33]. Several alignment construction, and tables presenting the missing alternative scenarios to the chromalveolate hypothesis data information and descriptions of all genes (electronic have also been formulated, all attempting to explain the supplementary material, table S1 and S4, respectively). data by suggesting that plastids in ‘chromalveolate’ lineages originated through a single secondary endosymbiosis (b) Phylogenetic analyses within a subgroup of chromalveolates, and then spread The fit on the data of two evolutionary models—the site- to other subgroups by multiple tertiary endosymbioses homogeneous LG model and the site-heterogeneous mixture [34–36]. In these complex alternatives, haptophytes and CAT model—was evaluated by cross-validation (CV) as cryptophytes emerged as key players in early plastid dis- implemented in PHYLOBAYES v. 3.3b [40]. A learning set and a semination. Cryptophytes are also peculiar in that they test set were generated by randomly splitting (without replace- are the only known lineage that still harbours the red algal ment) the original alignment into 10 replicates made of 90 per endosymbiont nucleus (the nucleomorph), making them cent (50 293 amino acids) and 10 per cent (5588 amino acids) of pivotal significance to study endosymbiosis [37]. In of the original sites, respectively. For each replicate, a Markov addition, haptophytes include some of the most successful chain Monte Carlo run was then performed for a total of 5000 marine primary producers, which have a profound impact cycles (CAT) or 1100 cycles (LG), the lower number of on global biogeochemical equilibria [38]. Despite the cycles under LG being due to a much greater computational ongoing interest in these organisms, their phylogenetic pos- time per cycle. The first 1000 and 100 points were discarded ition remains among the most uncertain of any eukaryote. as burn-in for the CAT and LG runs, respectively, and the Here, we have addressed this problem by deep- remaining points used to compute the cross-validation log-like- sequencing a cDNA library for the last hacrobian taxon lihood. Bayesian inferences using the best tested model (CAT) in for which no genome-wide sequence data are available, combination with four gamma categories for handling the rate the katablepharid Roombia truncata, and at the same heterogeneity across sites (G4)wereperformedinPHYLOBAYES time considerably extended the length of previously pub- v. 3.3b for all datasets. In each analysis, two independent lished alignments by taking advantage of newly sequenced chains were run for a minimum of 5000 cycles or until conver- genomes (in particular that of the cryptophyte Guillardia gence of the chains was reached, removing the first 1000 theta). We assembled a supermatrix to test the hacrobian cycles as burn-in and calculating the posterior consensus on question, consisting of 258 genes and 68 carefully selec- the remaining trees. The Dayhoff 6 classes was used for recoding ted operational taxonomic units (OTUs), and used it to the amino acids, and analysed as above. Convergence between assess the monophyly of Hacrobia and the relationships the chains was ascertained by examining the difference in fre- of its constituent lineages to other eukaryotes. quency for all their bipartitions (less than 0.15 in all analyses). Bootstrap CAT proportions were computed with 100 pseudo- replicates generated with SEQBOOT from the PHYLIP v. 3.69 pack- 2. MATERIAL AND METHODS age [41], and run for 5000 cycles under the CAT þ G4model Roombia truncata culturing and cDNA construction, sequen-
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