Haptophyte algae (Haptophyta) Linda K. Medlin clock. Species identiA cation within Haptophyta is largely Marine Biological Association of the UK, The Citadel, Plymouth PL1 based on scale morphology and oJ en requires electron 2PB, UK ([email protected]) microscopy. Two molecular clocks have been made for the hapto- phytes by Medlin and her coworkers: a strict molecular Abstract clock using the Lintree program that averages the rate of Haptophytes are members of the marine phytoplank- evolution across all lineages (5, 6) and a relaxed molecu- ton involved in many important biochemical cycles. They lar clock (r8s) where the rate of evolution is allowed to possess two smooth fl agella and another organelle, called vary across the lineages (7, 8). Both clocks were cali- a haptonema inserted between the fl agella. The cells are brated using at least three calibration points from the covered by organic scales, which are calcifi ed in one order, coccolith fossil record: the character-based constraint the Coccolithales, permitting molecular clock calibration. of 195 Ma for the emergence of all coccolithophores, Time estimates place the divergence of the two classes and the divergence-based constraints of 64 Ma for the in the Neoproterozoic, ~800 million years ago (Ma), with divergence of Coccolithus from Cruciplacolithus and order-level diversifi cation occurring in the Phanerozoic, ~340–120 Ma. Selective survival of different orders across major extinction events may be related to the ability of the cells to switch their mode of nutrition from autotrophy to mixotrophy. Haptophytes (Fig. 1) occur in all seas and are oJ en major components of the nanoplankton (1, 2). 7 ey are import- ant primary producers, and some species in the genera Emiliania, Gephyrocapsa, Phaeocystis, Chrysochromulina, and Prymnesium may form extensive blooms with major biogeochemical, ecological, or economic impact. Most species are marine, but a few thrive in freshwater. Most are unicellular, planktonic biP agellates, but palmelloid, coccoid, amoeboid, colonial, and benthic forms also occur (3). Nearly all are photosynthetic, but phagotrophy and mixotrophy appears to be common in some genera (e.g., Chrysochromulina) (4). In most species, at least one stage in their haplo-diplont life cycle possesses two P a- gella that are similar in form and have no tubular hairs. Between the P agella is a unique organelle, called a hap- tonema, which diB ers structurally from the P agellum. Its length varies and it has been secondarily lost in some species. It can coil or bend, but not beat, and can attach Fig. 1 Chrysochromulina (Prymnesiales) with arrow indicating to a substratum and may be involved in food handling. long haptonema (upper left), Phaeocystis (Phaeocystales) with arrow indicating short haptonema (upper right), colony Cells are typically covered by one to several layers of of Phaeocystis antarctica (lower left), and coccolithophore organic scales and in the coccolithophorids these are cal- Emiliania huxleyi (lower right). Credits: W. Eikrem (upper left), ciA ed. 7 ese are preservable and constitute the feature L. K. Medlin (upper right and lower left), and J. Green (lower that leaves a fossil record for calibration of a molecular right). L. K. Medlin. Haptophyte algae (Haptophyta). Pp. 123–126 in e Timetree of Life, S. B. Hedges and S. Kumar, Eds. (Oxford University Press, 2009). HHedges.indbedges.indb 112323 11/28/2009/28/2009 11:25:36:25:36 PPMM 124 THE TIMETREE OF LIFE Isochrysidales 5 Coccolithales 4 Prymnesiales-4 7 Prymnesiales-3 6 Prymnesiales-2 2 8 Prymnesiales-1 Phaeocystales-2 Prymnesiophyceae 1 9 Phaeocystales-1 Pavlovales-2 3 Pavlovales-1 Neoproterozoic Paleozoic Mesozoic Cz Pavlovophyceae PROTEROZOIC PHANEROZOIC 800 600 400 200 0 Million years ago Fig. 2 A timetree of Haptophyta. Divergence times are shown C. parva; Prymnesiales-2 = Clade B2 other saddle-shaped in Table 1. Pavlovales-1 = Exanthemachrysis with pigment type A Chrysochromulina sp.; Prymnesiales-3 = Clade B1 Imantonia (15); Pavlovales-2 = Pavlova and Diacronema with pigment type sp. and OLI clones; and Prymnesiales-4 = Clade B1 round, not B and C (15); Phaeocystales-1 = unicellular Phaeocystis sp. (6); saddle-shaped Chrysochromulina sp., Prymnesium sp., and Phaeocystales-2 = colonial Phaeocystis sp. (6); Prymnesiales-1 = Platychrysis sp. Details of species in each clade are presented Clade B2 saddle-shaped Chrysochromulina sp. including elsewhere (1). Abbreviation: Cz (Cenozoic). 50 Ma for the divergence of Helicosphaeraceae from thus this divergence appears to be a late Paleozoic–early Pontosphaeraceae. We have constructed a molecu- Mesozoic event and may be associated with the Permian– lar clock from the small subunit ribosomal RNA (SSU Triassic boundary (251 Ma). Modern diversiA cations in rRNA) gene and extrapolated dates for some of the these lineages occurred some time aJ er the lineage ori- undated nodes where there is no fossil evidence. 7 e gin so many taxa were presumably lost during this time. SSU rRNA tree appears to be evolving in a clocklike Within the Order Phaeocystales, the divergence of manner as judged by the relative rate tests performed in the cold water clades from the warm water clades occurs addition to the use of the Lintree program (9). Another at 30 Ma, when the Drake Passage opened to isolate the molecular clock study of haptophytes has been done, Antarctic Continental waters, and dispersal to the Artic using the SSU and large subunit (LSU) rRNA genes occurred across the equator during a cooling trend at (10). Dates for the divergences in that study are slightly 15 Ma, which were separated by a warming trend that older than those found by Medlin and coworkers using then isolated the two polar clades (6). a relaxed molecular clock (8). Molecular diversiAcation occurred earlier within Although not treated in detail here, Haptophyta is a the Prymnesiales than within the Coccolithales group that diverged from other eukaryotes deep in the plus Isochrysidales where most of these latter diver- Proterozoic, >1200 Ma (9–11). 7 e long time period gences occurred fairly late in the haptophyte timetree between the origin of haptophytes and the initial diver- (Fig. 2). 7 e diversiA cation within the Coccolithales gence (~800 Ma) of the two classes, Pavlovophyceae and plus Isochrysidales occurred predominantly aJ er the Prymnesiophyceae (Table 1, Fig. 2), indicates that many of Mesozoic–Cenozoic boundary (66 Ma), as predicted the early evolutionary branches in this group are extinct, by the fossil record. Mesozoic coccolithophores have or that they have not yet been sampled (2). 7 e Order been intensively studied and at the Mesozoic–Cenozoic Phaeocystales diverged from all other Prymnesiophyceae boundary an abrupt extinction is documented in the at ~480 Ma and then the Prymnesiales diverged from fossil record with ~90% of end-Cretaceous species dis- the Coccolithales plus Isochrysidales at ~280 Ma, and appearing (e.g., 12, 13). Subsequently, there was a major HHedges.indbedges.indb 112424 11/28/2009/28/2009 11:25:39:25:39 PPMM Eukaryota; Haptophyta 125 Table 1. Divergence times among haptophytes. Timetree Estimates Node Time Ref. (8) Ref. (9) Ref. (10)(a) Ref. (10)(a) Time Time Time Time 1 800 800 500 870 1000 2480480200400290 3 280 280 – 300 330 4280280– 300210 5200200– 250150 6200200– 180150 7175175– – – 8150150– – – 9 120 120 – – – Note: The node times in the timetree are based on ref. (8). Estimates from ref. (10) are based on (a) SSU rRNA and (b) LSU rRNA analyses. radiation in the early Cenozoic with new clades rap- reverse is true, they engulf prey and survive heterotro- idly diversifying and forming the origins of the modern phically. At the Mesozoic–Cenozoic boundary, it is likely coccolithophore biota (e.g., 14). that light quality was reduced and photosynthetic abil- One novel inference from our molecular tree is that ity was impaired. 7 erefore, those taxa with either the the Mesozoic–Cenozoic boundary extinction does not ability to form resting stages, such as the diatoms and seem to have aB ected the Prymnesiales, Phaeocystales, the dinoP agellates, or the ability to switch their mode of or Pavlovales to the same degree as the Coccolithales. nutrition could have an adaptive advantage over those 7 ese orders do not have a fossil record so we can only that did not have either of these traits. Coccolithophores make this statement by comparing the depth of clade are not known to form resting stages, in the strictness diversiA cation. In each of these noncalcifying groups, sense, and it appears that they are predominantly obli- there are numerous clades/lineages that cross the gate autotrophs. 7 us, at the Mesozoic–Cenozoic bound- Mesozoic–Cenozoic boundary (8). 7 ere is no evidence ary, the stress induced by reduced light quantity and of major diversiA cation of these clades in the Cenozoic. quality could have shut down photosynthesis. Cells that On this basis, one would expect that the noncalciA ed could switch nutrition or form resting stages would have haptophytes would have the same rate of extinction as had a better chance of survival. the calciA ed ones, because no group produces resting In summary, the haptophytes are a major eukary- stages, although some species have benthic littoral to otic group of microalgae whose closest relative is sublittoral stages as part of their dimorphic life cycle. No unclear. 7 e initial class level divergence occurred in haptophytes are known to produce specialized resting the Neoproterozoic and divergence of the orders appears cells or zygotes analogous to dinoP agellate cysts or dia- to be associated with the Permian–Triassic boundary. tom resting spores. 7 ere is no evidence of bottlenecking Because this is a host lineage with a red algal plastid, it in the noncalciA ed taxa at this time, as illustrated by the is likely that the group radiated at the Permian–Triassic many clades with deeper divergences (Fig.