Parasite Speciation Research Articles

The importance of host spatial distribution for parasite specialization and speciation: a comparative study of bird fleas (Siphonaptera: Ceratophyllidae)

Summary The environment of parasites is determined largely by their hosts. Variation in host quality, abundance and spatial distribution affects the balance between selection within hosts and gene flow between hosts, and this should determine the evolution of a parasite's host‐range and its propensity to locally adapt and speciate. We investigated the relationship between host spatial distribution and (1) parasite host range, (2) parasite mobility and (3) parasite geographical range, in a comparative study of a major group of avian ectoparasites, the birds fleas belonging to the Ceratophyllidae (Siphonaptera). Flea species parasitizing colonial birds had narrower host ranges than those infesting territorial nesters or birds with an intermediate level of nest aggregation. The potential mobility and geographical ranges of fleas decreased with increasing level of aggregation of their hosts and increased with the fleas’ host ranges. Birds with aggregated nest distribution harboured more flea species mainly due to a larger number of specialists than solitarily nesting hosts. These results emphasize the importance of host spatial distribution for the evolution of specialization, and for local adaptation and speciation in Ceratophyllid bird fleas.

Speciation in fig pollinators and parasites

Here we draw on phylogenies of figs and fig wasps to suggest how modes of speciation may be affected by interspecific interactions. Mutualists appear to have cospeciated with their hosts to a greater extent than parasites, which showed evidence of host shifting. However, we also repeatedly encountered a pattern not explained by either cospeciation or host switching. Sister species of fig parasites often attack the same host in sympatry, and differences in ovipositor length suggest that parasite speciation could result from divergence in the timing of oviposition with respect to fig development. These observations on fig parasites are consistent with a neglected model of sympatric speciation.

Ecology and biogeography of marine parasites

A review is given of (mainly recent) work on the biodiversity, ecology, biogeography and practical importance of marine parasites. Problems in estimating species numbers have been thoroughly discussed for free-living species, and the main points of these discussions are reviewed here. Even rough estimates of the richness of most parasite groups in the oceans are premature for the following reasons: species numbers of host groups, in particular in the deep sea and the meiofauna, are not known; most host groups have been examined only insufficiently for parasites or not at all; even in some of the best known groups, latitudinal, longitudinal and depth gradients in species richness are only poorly understood or not known at all; effects of hosts on parasite morphology and geographical variation have been studied only in a few cases; there are few studies using techniques of molecular biology to distinguish sibling species. Estimates of species richness in the best known groups, trematodes, monogeneans and copepods of marine fishes, are given. Parasites are found in almost all taxa of eukaryotes, but most parasitic species are concentrated in a few taxa. Important aspects of the ecology of marine parasites are discussed. It is emphasized that host specificity and host ranges should be distinguished, and an index that permits calculation of host specificity is discussed. The same index can be applied to measure site specificity. Central problems in ecology are the importance of interspecific competition and whether equilibrium or non-equilibrium conditions prevail. Marine parasites are among the few groups of organisms that have been extensively examined in this regard. A holistic approach, i.e. application of many methods, has unambiguously shown that metazoan ecto- (and probably endo-) parasites of marine fish live in largely non-saturated niche space under non-equilibrium conditions, i.e. they live in assemblages rather than in communities structured by competition. Nestedness occurs in such assemblages, but it can be explained by characteristics of the species themselves. There is little agreement on which other factors are involved in "structuring" parasite assemblages. Few studies on metapopulations of marine parasites have been made. A new approach, that of fuzzy chaos modelling, is discussed. It is likely that marine parasites are commonly found in metapopulations consisting of many subpopulations, and they are ideally suited to test the predictions of fuzzy chaos. Some recent studies on functional ecology and morphology--especially with regard to host, site and mate finding--are discussed, and attention is drawn to the amazing variety of sensory receptors in some marine parasites. Effects of parasites on hosts, and some studies on the evolution and speciation of marine parasites are discussed as well. A detailed overview of biogeographical studies is given, with respect to latitudinal gradients in species diversity, reproductive strategies and host ranges/specificity. Studies of marine parasites have contributed significantly to giving a non-equilibrium explanation for latitudinal diversity gradients. Recent studies on longitudinal and depth gradients are discussed, as well as parasites in brackish water, parasites as indicators of zoogeographical regions and barriers, and parasites as biological tags. The practical importance of marine parasites in mariculture, as monitors of pollution, agents of human disease, the use of parasites for controlling introduced marine pests, and some related aspects, are also discussed.

Biological evolution and statistical physics

This review is an introduction to theoretical models and mathematical calculations for biological evolution, aimed at physicists. The methods in the field are naturally very similar to those used in statistical physics, although the majority of publications have appeared in biology journals. The review has three parts, which can be read independently. The first part deals with evolution in fitness landscapes and includes Fisher's theorem, adaptive walks, quasispecies models, effects of finite population sizes, and neutral evolution. The second part studies models of coevolution, including evolutionary game theory, kin selection, group selection, sexual selection, speciation, and coevolution of hosts and parasites. The third part discusses models for networks of interacting species and their extinction avalanches. Throughout the review, attention is paid to giving the necessary biological information, and to pointing out the assumptions underlying the models, and their limits of validity.

Biological evolution and statistical physics

This review is an introduction to theoretical models and mathematical calculations for biological evolution, aimed at physicists. The methods in the field are naturally very similar to those used in statistical physics, although the majority of publications appeared in biology journals. The review has three parts, which can be read independently. The first part deals with evolution in fitness landscapes and includes Fisher's theorem, adaptive walks, quasispecies models, effects of finite population sizes, and neutral evolution. The second part studies models of coevolution, including evolutionary game theory, kin selection, group selection, sexual selection, speciation, and coevolution of hosts and parasites. The third part discusses models for networks of interacting species and their extinction avalanches. Throughout the review, attention is paid to giving the necessary biological information, and to pointing out the assumptions underlying the models, and their limits of validity.

The diversity of parasites.

Parasitism is one of the most successful modes of life displayed by living organisms, as measured by how often it evolved and how many parasitic species are presently in existence. Studying the diversity of parasites is particularly relevant because sympatric diversification may be important in some parasite taxa, and because of the opportunity for independent tests of evolutionary hypotheses in the many separate lineages in which parasitism evolved. Our incomplete knowledge of existing parasite species--the result of a range of phenomena that includes inadequate sampling effort or the lumping of different cryptic species under one name--is not always a major obstacle for the study of parasite diversity. Patterns in the diversity of parasites may be associated with either host or parasite characteristics. The distribution of parasite diversity among host taxa does not simply reflect the species diversity of the host taxa themselves; life history and ecological traits of hosts appear to play important roles. These may determine the likelihood that hosts are colonized by parasite species over evolutionary time. It is not yet clear whether some host traits also favor intrahost speciation and diversification of parasites, and the formation of new parasite species. Certain features of parasites may also be associated with speciation and diversification. Only parasite body size has received much attention; the patterns observed are not greatly different from those of free-living species, with small-bodied parasite taxa being more speciose than related large-bodied taxa. Epidemiological parameters such as the basic reproductive rate of parasites, or R0, can also generate predictions regarding the distribution or evolution of parasite diversity. For instance, parasite taxa characterized by high R0 values may be more speciose than related taxa with lower values of R0; such predictions remain untested. Large-scale biogeographical patterns of diversity have only been well studied for metazoan parasites of marine fish; for these parasites, latitudinal patterns can be explained by effects of temperature on speciation rates and epidemiological variables, though other causes are possible. The emphasis for future research must shift from pattern description to the elucidation of the processes responsible for the structure and diversity of parasite faunas. A better integration of ecological and historical (or phylogenetic) approaches to the study of parasite diversity should make this objective possible.

Imprinting and the origin of parasite–host species associations in brood-parasitic indigobirds, Vidua chalybeata

Brood-parasitic village indigobirds, Vidua chalybeata, were bred in captivity and foster-reared by their normal host species, the red-billed firefinch, Lagonosticta senegala, or by an experimental foster species, the Bengalese finch, Lonchura striata. Captive-reared female indigobirds were tested as adults for mate choice and for host choice. In tests of mate choice, female indigobirds responded preferentially towards mimicry songs of male indigobirds that were similar to those of the females' own foster parents. Females reared by Bengalese finches responded to male songs that mimicked Bengalese finch song rather than to male songs that mimicked their normal host species, the firefinch. In tests of host choice, females reared by Bengalese finches laid in the nests of Bengalese finches, and females reared by firefinches laid in the nests of firefinches. Wild-caught females showed the same behaviours as captive-bred females reared by firefinches. A female indigobird's social companions (firefinch or Bengalese) following her independence of her foster parents had no effect on her sexual response to male mimicry song or her choice of a host species in brood parasitism. The results support the predictions of a model of imprinting-like behaviour development in which young indigobirds focus their attention on their foster parents, rather than a model of innate bias for songs and nests of their normal host species, or a null model of nonspecific brood parasitism and differential survival. The results provide experimental support for the recent origin of brood parasite–host associations and the significance of imprinting in speciation in these brood parasites.

How Prevalant are Plasmodium ovale and P. malariae in East Asia?

Plasmodium ovale and Plasmodium malariae, two of the four human malaria parasites, are usually found at very low prevalence in East Asia, even in areas with intense malaria transmission. In this article, Fumihiko Kawamoto, Qing Liu, Marcelo Ferreira and Indah Tantular review data obtained in recent field surveys, using alternative diagnostic methods such as acridine orange staining and PCR-based methods, to evaluate the prevalence of these two malaria species in East Asia. They argue that these species might be much more prevalent in East Asia than reported previously. In addition, they discuss the implications of sequence variations found in the small subunit ribosomal RNA genes of the two species targeted by diagnostic PCR and compare morphological criteria for speciation of malaria parasites stained with Giemsa and acridine orange.

Speciation and diversification of parasite lineages: an analysis of congeneric parasite species in vertebrates

The evolutionary diversification of living organisms is a central research theme in evolutionary ecology, and yet it remains difficult to infer the action of evolutionary processes from patterns in the distribution of rates of diversification among related taxa. Using data from helminth parasite communities in 76 species of birds and 114 species of mammals, the influence of four factors that may either be associated with or modulate rates of parasite speciation were examined in a comparative analysis. Two measures of the relative number of congeneric parasite species per host species were used as indices of parasite diversification, and related to host body mass, host density, latitude, and whether the host is aquatic or terrestrial. The occurrence of congeneric parasites was not distributed randomly with respect to these factors. Aquatic bird species tended to harbour more congeneric parasites than terrestrial birds. Large-bodied mammal species, or those living at low latitudes, harboured more congeneric parasites than small-bodied mammals, or than those from higher latitudes. Host density had no apparent association with either measures of parasite diversification. These patterns, however, reflect only the present-day distribution of parasite diversification among host taxa, and not the evolutionary processes responsible for diversification, because the apparent effects of the factors investigated disappeared once corrections were made for host phylogeny. This indicates that features other than host body size, host density, latitude, and whether the habitat is terrestrial or aquatic, have been the key driving forces in the diversification of parasitic helminth lineages.

Correlation of parasite speciation and specificity with host evolutionary relationships

Protopolystoma (Monogenea, Polystomatidae) is strictly specific to the anuran amphibian genus Xenopus. The host group is characterised by a polyploid series in which chromosome numbers reflect diploid, tetraploid, octoploid and dodecaploid constitutions; the series is considered to have evolved through interspecies hybridisation and genome duplication. This study correlates information on host evolutionary relationships with patterns of parasite speciation and host specificity. Protopolystoma is restricted to one subgenus ( Xenopus) with multiples of 36 chromosomes, and is absent from the subgenus Silurana (with multiples of 20 chromosomes). Molecular, biochemical and karyotype evidence distinguishes three subgroups within Xenopus. Representative species from each subgroup, Xenopus muelleri, Xenopus fraseri and Xenopus laevis, have been examined for polystomatid infection. Two species of Protopolystoma occur in each of these host species. In X. muelleri, the two Protopolystoma species reflect parasite co-speciation corresponding with the divergence of two sibling host species. Xenopus fraseri and X.laevis (both with 2n=36 chromosomes) are implicated in the hybrid origin of two octoploid species, Xenopus wittei and Xenopus vestitus (both 2n=72). The relationships of the Protopolystoma species in these Xenopus taxa reflect this presumed ancestry. Xenopus wittei carries two species of Protopolystoma, one shared with X.fraseri and the other shared with X. laevis. Xenopus vestitus carries a single species of Protopolystoma which is shared with X. laevis but there is no “heirloom” which reflects its hybrid origin involving X. fraseri. In addition to these shared parasite species which may reflect shared host genes, X. fraseri and X. laevis each carry separate species-specific Protopolystoma which do not occur in other Xenopus species even where there is evidence of common genetic information (as in the allopolyploid wittei and vestitus). This case study may be interpreted as indicating a powerful influence of host genetic factors on susceptibility to infection, host-specificity, and parasite speciation.

Parasite speciation within or between host species?—Phylogenetic evidence from site-specific polystome monogeneans

Partial nuclear 28S ribosomal RNA and mitochondrial cytochrome c oxidase subunit I (COI) gene sequences (953 and 385 nucleotides, respectively) of one fish monogenean (outgroup) and six polystome monogeneans (four Polystomoides spp. from the oral cavities and urinary bladders of freshwater turtles in Australia and Malaya, two Neopolystoma spp. from the urinary bladder and conjunctival sac of a freshwater turtle in Australia) were used to examine the question of whether congeneric species infecting different sites in the same host species have speciated in that host by adapting to different sites, or whether species infecting a particular site in one host have given rise to species infecting the same site in different hosts. Results show unequivocally that congeneric species infecting the same site, even of host species belonging to different suborders and occurring on different continents, are more closely related than congeneric species infecting different sites of the same host species. This is interpreted as meaning that speciation has not occurred in one host. Morphological evolution of polystomes has been very slow: few differences between species and even genera have evolved over a period of at least 150 Myr, and this is matched by low substitution rates of nucleotides, and the ambiguous position of species of different genera, depending on whether COI or 28S rDNA sequences are used.

Parasite faunas of freshwater fish: The relationship between richness and the specificity of parasites

The relationship between the host specificity of parasites and the richness of the assemblages in which they occur was examined among the parasite faunas of Canadian freshwater fishes. The prediction tested was that rich faunas would consist of both generalist and specialist parasites, whereas poor faunas would include only generalists, a pattern known as nestedness. An index of nestedness was computed for parasite faunas of fish species in 5 large families (Salmonidae, Cyprinidae, Catostomidae, Centrarchidae and Percidae) and compared with the value expected if parasite faunas are random assemblages of parasites. There was no evidence of nested patterns among any of the 5 families of fish hosts. However, since both measures of host range of parasites (i.e. number of known host species) and richness of parasite faunas are affected by how intensely the different parasites and hosts have been studied, tests of nestedness may be flawed. After correcting both variables for study effort, negative correlations were found between the mean host range of parasites and the richness of the faunas to which they belong. In other words, parasites in rich faunas occurred on average in fewer host species, because of the many specialists, than parasites in poor faunas, which are mainly generalists. This relationship was apparent in all fish but the Salmonidae; fish species in this family have been introduced to new areas much more frequently than other fish, and their parasite faunas have thus had a distinct recent history. The general trend observed in non-salmonid fish suggests that parasite colonization and speciation may have been facilitated in some fish species, but not in others.

Conservation of low-copy gene loci in Old World leishmanias identifies mechanisms of parasite evolution and diagnostic markers

Genome plasticity has been hypothesized to be a driving force behind parasite speciation. We have evaluated divergence in single and low-copy genes in terms of locus organization, chromosomal localization and gene expression in Leishmania infantum, L. major, L. tropica and three widely divergent geographic isolates of L. donovani. Seventeen genes of low to moderate copy number (1–4 copies/haploid genome) were analyzed to identify restriction fragment length polymorphisms (RFLPs) providing heritable markers distinguishing Old World (OW) leishmanias. These RFLP markers were conserved in parasite isolates from primary infections demonstrating their utility as diagnostic tools. The species designations established by RFLP analysis of field isolates was confirmed by use of monoclonal antibodies. All 17 genes were present in each OW leishmania analyzed except LSIP (A45), which was absent from L. infantum. The 17 genes were found to be distributed among 9 distinct chromosomes. However, in spite of variations in chromosome karyotypes among the various OW leishmanias, individual gene probes localized to a similar sized chromosome from each isolate. These observations coupled with a molecular tree derived from RFLP data suggest that the OW leishmanias comprise a monophyletic lineage, with species associated with cutaneous disease exhibiting the greatest level of divergence. Data from this study supports previous observations that species causing cutaneous and visceral disease have diverged primarily by nucleotide substitutions. Such nucleotide divergence may not only lead to changes in protein function and antigenicity, but may also alter gene regulation programs as exemplified by the finding that the LdI-9-5 and LdE-6-1 genes were expressed only in visceralizing leishmanias.

Morphology and cytology study of Foleyella agamae complex (Nematoda: Filarinae) infecting the agamid lizard Agama agama in Nigeria.

Three cytotypes of Foleyella agamae were isolated from naturally infected wild-caught Agama agama in Nigeria. Cytotype A (2n = 8) has a body length/oesophageal length ratio (L/O) of 87.6 +/- 14.2 in female worms and a large somatic size (length, 63.8 +/- 12.7 mm in female worms and 25.5 +/- 3.4 mm in males). Cytotype B (2n = 6) has an L/O of 43.4 +/- 6.6 in females and a small to average somatic size (length, 36.7 +/- 11.9 mm in females and 19.5 +/- 2.3 mm in males). Cytotype C (2n = 4) has an L/O of 23.8 +/- 8.2 in female worms and a small somatic size (length, 18.8 +/- 3.8 mm in females and 14.1 +/- 3.0 mm in males). No cytotype with odd numbers of chromosomes was observed. Comparisons with similar types of speciation in medically important filarial parasites were drawn. Introgressive hydridisation in the speciation of filarial parasites is postulated.

Historical biogeography and modes of speciation across high-latitude seas of the Holarctic: concepts for host – parasite coevolution among the Phocini (Phocidae) and Tetrabothriidae (Eucestoda)

Species of Anophryocephalus are host-specific parasites of pinnipeds in the Holarctic. Phylogenetic analysis of 7 species postulates A. anophrys as the basal taxon and A. inuitorum as basal to A. skrjabini; A. arcticensis is basal to A. nunivakensis and A. eumetopii + A. ochotensis (single tree; consistency index = 74.4%; homoplasy slope ratio = 36.45%). Evaluation of host and geographic distributions postulates ringed seals of the Atlantic–Arctic as ancestral hosts, and the Arctic basin as a paraphyletic area with respect to the North Pacific. Cospeciation within this assemblage was dependent on intense isolation of small effective populations of definitive hosts during the late Tertiary and Pleistocene glacial stages. Rapid modes of parasite speciation, compatible with microallopatry and peripheral isolation, appear to have been associated with isolation of pinniped populations in refugial habitats of the Arctic basin and Beringia. The biogeography of host–parasite assemblages among pinnipeds and Alcidae (Charadriiformes) during the Pliocene and Quaternary contrasts in part with the history elucidated for some free-living invertebrate taxa in the Arctic basin.

Social parasites in polistine wasps are monophyletic: implications for sympatric speciation

A social parasite uses workers of another social insect species to rear its own progeny. They are often so closely related to their hosts that it has been suggested that they could have evolved sympatrically from them. To address the question of whether social parasites evolved from their hosts we present a partial sequence of the mitochondrial 16 S rRNA gene for nine species of Polistes , comprising all known species of social parasites, their hosts and two outgroups. Parsimony and maximum-likelihood analyses of the data support monophyly for these social parasites. The trees supporting monophyly are significantly shorter than the trees supporting sympatric speciation of parasites from their hosts. These data support the hypothesis that speciation occurred allopatrically and independently of the evolution of social parasitism. Where the social parasite parasitizes more than one species, the two species used are most closely related to each other. Although social parasites are monophyletic and did not evolve sympatrically from their hosts, it is clear that relatedness among species is important in the host–parasite relation.

Apparent lack of genetic variation within Pelecitus roemeri (Nematoda: Filarioidea) from three Australian species of macropodid marsupial

An electrophoretic study of Pelecitus roemeri from Macropus robustus, M. giganteus and Wallubia bicolor revealed no genetic differences at 23 enzyme loci. The genetic data support the existing morphological evidence that P. roemeri from these three hosts represents a single species. The data show no genetic variation between nematodes from the same or different host species collected in northern and southern Australia. This result is discussed briefly in relation to Price's model of parasite speciation.

DNA analysis in the diagnosis of infection and in the speciation of nematodes parasites

DNA analysis is playing an increasingly important role in characterising and classifying nematode parasites. Though less emphasis has been placed on utilising DNA elements to study nematodes of veterinary importance, correct diagnosis is, nevertheless, critical to proper treatment and control. The genus Trichinella presents a particularly interesting problem since the level of classification within this genus remains unclear. Herein we discuss the application of DNA analysis to the diagnosis and speciation of the parasitic nematode Trichinella spiralis, where the study of cloned repetitive DNA elements and ribosomal RNA genes has led to significant advances in the understanding of the species-level systematics of this nematode. Despite advances in the recognition of multiple gene pools within the genus Trichinella, their taxonomic level is still uncertain because of insufficient knowledge regarding intraspecific variation within this genus. The utility of mitochondrial DNA and ribosomal DNA sequence analysis in studying the phylogeny and evolutionary history of parasites within this genus is also discussed.

Gene frequency clines for host races of Rhagoletis pomonella in the Midwestern United States

Speciation in many host specific parasites may be initiated in sympatry when populations shift and adapt to new hosts. The recent shift of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae) from its native host plant hawthorn (Crataegus spp.) to introduced, domesticated apple (Malus pumila), provides a direct test of the “sympatric speciation” hypothesis by indicating whether partially reproductively isolated “host races” can evolve in the absence of geographic isolation. We report finding significant allele frequency differences for six allozymes between paired apple and hawthorn infesting populations of R. pomonella from across the midwestern United States. Latitudinal allele frequency clines exist among both apple and hawthorn populations, however, for a majority of the loci displaying racial differences. Inter-host genetic differentiation is therefore superimposed on clinal patterns of variation within the races such that the magnitude of host associated divergence is a function of latitude. The results indicate that host associated races can form in sympatry and implicate differences in host plant recognition and developmental timing (related to ambient temperature) as key factors restricting gene flow between apple and hawthorn populations. However, some of the same processes differentiating apple and hawthorn populations at sympatric sites also appear to be occurring within the two host races across their respective ranges. R. pomonella populations are therefore diverging with respect to both their host plant affiliations and local environmental conditions.

Prospects in Systematics.

Part 1 Introduction: systematics, D.L.Hawksworth, F.A.Bisby (Department of Biology, University of Southampton) recent historical developments, E.Mayr (Museum of Comparative Zoology, Harvard University) the structure of systematics, V.H.Heywood (Department of Botany, University of Reading). Part 2 Systematics and evolution: the impact of evolutionary theories on systematics, C.Patterson (British Museum) species concepts and speciation in parasites, M.F.Claridge (Department of Zoology, University College, Cardiff) the detection of phylogeny, J.Felsenstein (Department of Genetics, University of Washington) the contribution of palaeontology to systematics and evolution, A.Hallam (Geology Department, University of Birmingham). Part 3 Implications of modern methods: the new genetics, G.A.Dover (Department of Genetics, University of Cambridge) the estimation of protein and nucleic acid homologies, J.P.W.Young (John Innes Institute, Norwich) isozyme studies and their interpretation, A.Ferguson (Department of Zoology, Queen's University, Belfast) some implications of the revolution in molecular biology, K.A.Joysey (University Museum of Zoology, Cambridge) electron microscopy in systematics - genesis and revelations, J.G.Duckett (Botany Department, Queen Mary College, London) numerical techniques as an aid to objectivity, J.C.Gower (Statistics Department, Rothamsted Experimental Station, Harpenden) the phenetic and cladistic approaches, P.H.A.Sneath (Department of Microbiology, University of Leicester). Part 4 Taxonomic information systems: communications in taxonomy, F.A.Bisby automated taxonomic descriptions, L.Watson (Research School of Biological Sciences, Australian National University) et al the uses and future of bibliographic database systems, S.G.Dextre Clarke (CAB International, Slough) taxonomically intelligent database programs, R.Allkin (Royal Botanic Gardens, Kew) towards a unified system of biological nomenclature, W.D.L.Ride (School of Applied Science, Canberra College of Advanced Education). Part 5 Systematics and applications: exploitation of taxonomic databases in selective isolation programmes, R.D.Nolan (ICI Pharmaceuticals Division, Macclesfield) the needs of plant breeding, agriculture and horticulture, B.Pickersgill (Department of Agricultural Botany, University of Reading) the demands of medicine and veterinary science, R.L.J.Muller (CAB International Institute of Parasitology, St Albans), J.R.Baker. Part 6 Prospects for the next century: user needs in systematics and obstacles to their fulfilment, P.T.Haskell (University College, Cardiff), P.J.Morgan (National Museum of Wales, Cardiff) systematics in the education of biologists, R.J.Berry (Department of Zoology, University College, London) the view from the origin and early evolution of life - secession of the Protoctista from the animal and plant kingdoms, L.Margulis (Department of Biology, Boston University). Index. Systematics Association publications.