CODENET WORK AREA 2 - MICROEVOLUTION AND SPECIES LEVEL VARIATION

Biodiversity occurs at all evolutionary scales and our investigation of high level diversity will be complimented by study of microevolution and species level variation. This work will be primarily focused on the genus Gephyrocapsa, a plexus of recently diversified morphospecies for which extensive background studies are already available. Similar studies, if less extensive, studies will also be carried out on the other species, each of which presents particular problems. Taken together these should constitute a sufficiently large number of case studies to provide a general model of the nature of species level variation within the group. This page outlines the background, methodology, and objectives of this work area.

BACKGROUND

Low-level taxonomy in both living and fossil coccolithophorids is based exclusively on coccolith morphology, and at species level on fine variations in size and shape (Jordan & Green 1994). Studies of cultured coccolithophorids (e.g. Inouye & Pienaar 1984, 1988, Fresnel & Billard 1991, Young & Westbroek 1991) support the assumption that aspects of coccolith morphology are stable and under strong genotypic control: morphological variation observed in culture is within the bounds of natural variability.

Emiliania huxleyi is the only coccolithophorid for which variability has been intensively studied, combining quantitative observation on variation in culture and in natural oceanic populations (e.g. Watabe & Wilbur 1966, McIntyre & Be 1967, Young & Westbroek 1991, Young 1994, Paasche et al. 1996, Medlin et al. 1996). These results suggest that fine-scale genotypic heterogeneity occurs within oceanic populations, probably related to genotypic recombination during the haplo-diploid life-cycle. This fine-scale variability has some effect on ecological adaptation but minimal effect on coccolith morphology. At a slightly higher scale stable genotypic variation can be identified which is reflected in coccolith morphology defining sub-species which do not usually co-occur but which each have near global distribution.

In contrast, extensive geological studies of coccolith evolution including particularly morphometric studies of Gephyrocapsa (Samtleben 1980, Matsuoka & Okada 1990, Bollmann, 1997) suggest gradualistic evolution within global populations. However, without detailed parallel studies of morphological variation in living oceanic and culture material, observed morphological variation in fossil assemblages remains ambiguous.

Our work will exploit the exceptional possibilities offered by coccolithophorids for combined geological, biogeographical and cell-biological studies of species level variation. The selected species, display a diverse range of styles of morphological variation, and studies on them, will produce a sufficient number of case studies to allow general patterns to be developed. The Gephyrocapsa plexus will be studied in special detail since it represents a rapidly evolving group of closely related species. We will be able to compare microevolution in this genus from geological and biological perspectives.

METHODOLOGY

Research methods for study of species level biodiversity and microevolution are in part a direct continuation of those used for high-level evolutionary biodiversity, and the special study of Gephyrocapsa contributes to both work areas. There is, however, a shift here toward quantitative study of continuous variation, particularly of morphology. The main types of study to be integrated are:

1. Morphometric work on coccoliths in modern filter and sediment trap samples and in Holocene sediments, in order to characterise morphotypes present and to map out their distribution relative to ecological controls.

2. Study of culture isolates to determine the biological significance of morphotypic variation. Work in this area will include: Molecular genetic study of diversity between strains using fast-evolving non-coding regions or microsatellites; Study of lipid diversity; Study of physiological ecology; Study of pigment variability; Coccolith morphology. For, Gephyrocapsa we will isolate enough strains to study the full range of morphotypes. For the other species this cannot be guaranteed, but even study of variation within a single strain can be sufficient to indicate which aspects of morphological variation are stable and so which morphotypes are most likely to be the product of genotypic variation.

3. Study of geological time series in order to determine the evolutionary development and distribution of morphotypes. For this work a set of Deep Sea Drilling Project and Ocean Drilling Project cores will be used.

For the geological and biogeographical studies the different teams involved (NHM, ETHZ, FdA-VUA, MNHN-UL) will study different taxa but use the same samples. This approach will maximise the efficiency of the study in terms of study of background parameters, sample acquisition etc. It also will allow direct comparison of results between taxa and so allow testing of whether putative evolutionary events occur simultaneously in different lineages. These studies will be based on image analysis via video capture of light and electron microscope images. Specialised coccolith morphometrics applications have been developed for SEM by ETHZ (Bollmann 1995 , 1997) and for light microscopy by the NHM (Young et al. 1996). Training in, and development of, these applications will be provided by these teams for the network.

OBJECTIVES

Our basic objectives, for each of the keystone species, are to:

• 1. Determine ecophenotypic and genotypic variability using culture studies of individual strains, including analysis of: Physiological adaptation - growth rates under standard conditions and optimal growth conditions in terms of temperature and salinity. (RT9; NHM, ETHZ, CSIC); Coccolith morphology. (RT10; NHM, ETHZ, FdA-VUA, MNHN-UL); Lipid biomarker composition, including calibration of Uk37 palaeotemperature correlations. (RT4B,C; NIOZ, CSIC); Molecular genetic diversity using microsatellite probes. (RT3D: AWI); Photosynthetic pigment composition, HPLC analysis of variability in proportions of the major pigment components under varying light conditions. (RT5; CSIC)
• 2. Characterise the biogeographic distribution of intraspecific morphotypes, using modern and Holocene sample sets. (RT11-13; ETHZ, FdA-VUA, MNHN-UL)
• 3. Investigate the microevolutionary development of morphotypes, using geological sample sets. (RT14; NHM, ETHZ, MNHN-UL)

KEY SYNTHETIC OBJECTIVES ARE TO:

• Evaluate which aspects of variation represent genotypic vs. ecophenotypic or ontogenetic variation.
• Determine whether intra-specific variability in morphology, physiology and biochemistry are correlated, defining discrete sub-species.
• Determine whether physiological adaptation occurs within local sub-populations independently of other genotypic characters.
• Determine whether microevolution occurs by (sub-)species selection or effectively sympatric evolution within ocean-scale populations.