INA8

INA8
8^th International Nannoplankton Association Conference

ABSTRACTS

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Jeremy R. Young, Chantal Billard, Paul Bown, Lluïsa Cros, Sean Davis, Markus Geisen, Masanobu Kawachi, Steve Mann, Linda Medlin, Ian Probert, Alberto Saez:
Phylogeny of coccolithophorids and the evolution of calcification
(Talk)

Traditionally, knowledge of coccolithophorid phylogeny has been entirely dependant on stratophenetic data and, because this is by no means complete, there has been considerable uncertainty about interrelated fundamental questions, such as: the relationship of holococcolithophorids to heterococcolithophorids; the relationship of coccolithophorids to other haptophytes; whether coccolithophorids constitute a monophyletic group; and whether calcification has evolved repeatedly or only once.

Over the past decade, and especially the last few years, new data has become available from a range of sources enabling a new synthesis to be proposed. Key lines of information include:

Biomineralisation studies Young et al. (1992) demonstrated that many heterococcoliths are characterised by a highly distinctive biomineralisation mode commencing with nucleation of alternating subvertical and subradial calcite crystals, and hypothesised that this was a conserved mode indicating a common phylogenetic origin for all heterococcolithophorids. This hypothesis has been strongly supported by subsequent research (e.g. Young et al., 1999; Marsh et al., 1999). Moreover, Young et al. (1999) argued that the radically different holococcolith biomineralisation mode was equally distinctive and implied a common phylogenetic origin for all holococcolithophorids.

Molecular genetics Molecular phylogenies of haptophytes are rapidly being developed (Fujiwara et al., 1994; Inouye, 1997; Medlin et al., 1997; Edvardsen et al., 2000). These strongly indicate: (1) that coccolithophorids form a discrete clade within the haptophytes (including a few species which have secondarily lost the ability to calcify); (2) that the major diversification of the class Prymnesiophyceae occurred slightly before the diversification of the coccolithophorids (Medlin et al., 1997), which is known from stratophenetic data to have occurred in the Early Jurassic, ca.200Ma, (Bown, 1987); and (3) that the origin of the Haptophyta, and the subsequent divergence of the classes Pavolovophyceae and Prymnesiophyceae, occurred much earlier, probably in the Early Palaeozoic (ca.850Ma, 420Ma, respectively: Medlin et al., 1997).

Life-cycle studies It remains true that life-cycles have only been documented in a few haptophytes and very few coccolithophores. Synthesis of the data by Billard (1994) lead to the prediction that haplo-diplontic life-cycles were common to haptophytes and that distinct periplast types were characteristic of the phases. In particular, it was predicted that heterococcoliths might consistently occur on diploid phases and holococcoliths on haploid phases. This has been supported by flow cytometry studies of Emiliania huxleyi (Green et al., 1996) and most recently Coccolithus pelagicus (Probert et al., unpubl.: see also Edvardsen et al., 1996, for documentation of the haplo-diplontic life-cycles in a non-calcifying haptophyte). The data on Coccolithus pelagicus is critical because it provides the first direct confirmation that holococcoliths are indeed produced by haploid phases.

Combination coccospheres The life-cycle data has been greatly supplemented by documentation of combination coccospheres including holococcoliths and heterococcoliths (Thomsen et al., 1991; Kleijne, 1992; Alcober & Jordan, 1997; Young et al., 1998; Cros et al., in press; Cortes, in press; Sprengel et al., in press; Cros et al., this vol.; Geisen et al., this vol.). This work strongly supports the hypothesis that holococcoliths and heterococcoliths are alternate life-cycle phases and, moreover, indicates that in some species nannoliths substitute for holococcoliths.

Synthesis From this evidence, it appears likely that that the major diversification of haptophytes occurred in the Early Mesozoic and that a key event within this was the acquisition of calcification. We hypothesise this was a unique event and that calcification was subsequently passed from the haploid to the diploid phase (or vice versa). This implies that acquisition of calcification is a more significant evolutionary step than has previously been assumed. In addition, the differential expression of calcification in two phases of the life-cycle of a single organism provides an unique opportunity for studying the functional genomics and biochemistry of biomineralisation.

References

Alcober, J. & Jordan, R.W. 1997. An interesting association between Neosphaera coccolithomorpha and Ceratolithus cristatus (Haptophyta). European Journal of Phycology, 32: 91-93.

Billard, C. 1994. Life cycles. In: J.C. Green & B.S.C. Leadbeater (Eds). The Haptophyte Algae. Systematics Association Special Volume, 51: 167-186.

Bown, P.R. 1987. Early Mesozoic calcareous nannofossils. Special Papers in Palaeontology, 38: 1-118.

Cortes, M.Y. In press. Further evidence for the heterococcolith-holococcolith combination Calcidiscus leptoporus-Crystallolithus rigidus. Marine Micropaleontology, (INA7 Proceedings Special Issue), 39.

Cros, L., Kleijne, A., Zeltner, A., Billard, C. & Young, J.R. In press. New examples of holococcolith-heterococcolith combination coccospheres and their implications for coccolithophorid biology. Marine Micropaleontology, (INA7 Proceedings Special Issue), 39.

Cros, L., Kleijne, A. & Young, J.R. 2000. Coccolithophorid diversity in the genus Polycrater and possible relations with other genera. JNR, Abstracts INA8, 22(2).

Edvardsen, B., Eikrem, W., Vaulot, D. & Paasche, E. 1996. Comparison between the authentic and alternate Chrysochromulina polylepis: morphology, growth, taxonomy and ploidy level. In: T. Yasumoto, Y. Oshima & Y. Fukuyo (Eds). Harmful and toxic algal blooms. IOC UNESCO: 231-234.

Edvardsen, B., Eikrem, W., Green, J.C., Andersen, R.A., Moon-van fer Staay, S. & Medlin, L.K. 2000. Phylogenetic reconstructions of the Haptophyta inferred from 18S ribosomal DNA sequences and available morphological data. Phycologia, 39: 19-35.

Fujiwara, S., Sawada, M., Someya, J., Minaka, N., Kawachi, M. & Inouye, I. 1994.

Molecular phylogenetic analysis of rbcL in the Prymnesiophyta. Journal of Phycology, 30: 863-871.

Geisen, M., Cros, L., Probert, I. & Young, J.R. 2000. Life-cycle associations involving pairs of holococcolithophorid species: complex life cycles or cryptic speciation? JNR, Abstracts INA8, 22(2).

Green, J.C., Course, P.A. & Tarran, G.A., 1996. The life-cycle of Emiliania huxleyi: A brief review and a study of relative ploidy levels analysed by flow cytometry. Journal of Marine Systems, 9: 33-44.

Inouye, I. 1997. Systematics of Haptophyte algae in Asia-Pacific waters. Algae (The Korean Journal of Phycology), 12: 247-261.

Kleijne, A. 1991. Holococcolithophorids from the Indian Ocean, Red Sea, Mediterranean Sea and North Atlantic Ocean. Marine Micropaleontology, 17: 1-76.

Marsh, M.E. 1999. Coccolith crystals of Pleurochrysis carterae: crystallographic faces, organisation and development. Protoplasma, 207: 54-66.

Medlin, L.K., Kooistra, W.H.C.F., Potter, D., Saunders, G.W. & Andersen, R.A. 1997.

Phylogenetic relationships of the "golden algae" (haptophytes, heterokont chromophytes) and their plastids. Plant systematics and evolution (suppl.), 11: 187-219.

Sprengel, C. & Young, J.R. In press. First direct documentation of associations of Ceratolithus cristatus ceratoliths, hoop-coccoliths and "Neosphaera coccolithomorpha" planoliths. Marine Micropalaeontology, (INA7 Proceedings Special Issue), 39.

Thomsen, H.A., Østergaard, J.B. & Hansen, L.E. 1991. Heteromorphic life histories in arctic coccolithophorids (Prymnesiophyceae). Journal of Phycology, 27: 634-642.

Young, J.R., Didymus, J.M., Bown, P.R., Prins, B. & Mann, S. 1992. Crystal assembly and phylogenetic evolution in heterococcoliths. Nature, 356: 516-518.

Young, J.R., Jordan, R.W. & Cros, L. 1998. Notes on nannoplankton systematics and life-cycles. Ceratolithus cristatus, Neosphaera coccolithomorpha and Umbilicosphaera sibogae. Journal of Nannoplankton Research, 20(2): 89-99.

Young, J.R., Davis, S.A., Bown, P.R. & Mann, S. 1999. Coccolith ultrastructure and biomineralisation. Journal of Structural Biology, 126: 195-215.

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[Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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Tania Hildebrand-Habel (hiha@micropal.uni-bremen.de)