Chlamydomonas reinhardtii is a unicellular green alga whose lineage diverged from land plants over 1 billion years ago. It is a model system for studying chloroplast-based photosynthesis, as well as the structure, assembly, and function of eukaryotic flagella (cilia), which were inherited from the common ancestor of plants and animals, but lost in land plants. We sequenced the approximately 120-megabase nuclear genome of Chlamydomonas and performed comparative phylogenomic analyses, identifying genes encoding uncharacterized proteins that are likely associated with the function and biogenesis of chloroplasts or eukaryotic flagella. Analyses of the Chlamydomonas genome advance our understanding of the ancestral eukaryotic cell, reveal previously unknown genes associated with photosynthetic and flagellar functions, and establish links between ciliopathy and the composition and function of flagella.

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The Chlamydomonas Genome Reveals the Evolution of Key Animal and Plant Functions

Hybridization may influence evolution in a variety of ways. If hybrids are less fit, the geographical range of ecologically divergent populations may be limited, and prezygotic reproductive isolation may be reinforced. If some hybrid genotypes are fitter than one or both parents, at least in some environments, then hybridization could make a positive contribution. Single alleles that are at an advantage in the alternative environment and genetic background will introgress readily, although such introgression may be hard to detect. ‘Hybrid speciation’, in which fit combinations of alleles are established, is more problematic; its likelihood depends on how divergent populations meet, and on the structure of epistasis. These issues are illustrated using Fisher’s model of stabilizing selection on multiple traits, under which reproductive isolation evolves as a side-effect of adaptation in allopatry. This confirms a priori arguments that while recombinant hybrids are less fit on average, some gene combinations may be fitter than the parents, even in the parental environment. Fisher’s model does predict heterosis in diploid F1s, asymmetric incompatibility in reciprocal backcrosses, and (when dominance is included) Haldane’s Rule. However, heterosis arises only when traits are additive, whereas the latter two patterns require dominance. Moreover, because adaptation is via substitutions of small effect, Fisher’s model does not generate the strong effects of single chromosome regions often observed in species crosses.

The role of hybridization in evolution

(2002). The Effects of Harmful Algal Blooms on Aquatic Organisms. Reviews in Fisheries Science: Vol. 10, No. 2, pp. 113-390.

The Effects of Harmful Algal Blooms on Aquatic Organisms

THE STRUCTURE proposed for DNA (deoxyribonucleic acid) has been described before (for a rerien : see Jordan, 21) and will only be outlined here. It consists of two polynucleotide chains running in opposite directions and twined round one another. The two chains are held together by hydrogen bonds between the bases, each base being joined to a companion base on the other chain. This pairing of bases is specific, odenine going with thymine, and guanine wit.h cytosine. The structure is not only found in extracted DX4 from a wide variety of sources, but is also present in intact biological material such as sperm heads and bacteriophage (36). The x-ray work up to 1954 has already been briefly summarized, with references (6). Since then, the group at King’s College, London, under Dr. N. H. F. Wilkins have published an interim report (17) on their studies of the B form of DK-4 and on their work on nucleoprotamine. Two reviews (9, 37) have also appeared; the one by Wilkins (37) touching on the very recent work of his group on nucleohistone. Some studies of the B form of DB-4 have also been carried out by Wykoff (38), who has shown that the B structure obtained by stretching the -4 form is slightly different from that produced by swelling it. The King’s College group have obtained the B form in a crystalline state, and in particular, ha\-e shown that the lithium salt gives a good lattice (37). According to Wilkins (37:~. rhe structure is now firmly established. The detailed structure described by Crick and Watson (8) has been shown to have too large a diameter, and a drawing has been given of an improved model which is in fairly good agreement with the s-ray data. No coordinates ha\-? 50 fnr been published.* (It should be noted

The structure of dna

The role of polyploidy per se in the development of evolutionary novelty remains one of the outstanding questions in flowering plant evolution. Since chromosome doubling usually is associated with hybridization, the effects of doubling are difficult to uncouple from those of hybridity and recombination. Synthetic polyploids in crops typically are inferior to their diploid counterparts under conditions to which the diploids are adapted. This observation has suggested to many that chromosome doubling is a hindrance to progressive evolution. Evidence is presented herein from biochemical, physiological, developmental, and genetical sources which indicates that the nucleotypic effects of chromosome doubling are not necessarily negative. Indeed chromosome doubling may "propel" a population in to a new adaptive sphere, and render it capable of occupying habitats beyond the limits of its diploid progenitor. This postulate is consistent with what we know of the ecological tolerances of diploids and related polyplo...

Polyploidy and Novelty in Flowering Plants

Problems of cytology and evolution in the Pteridophyta

Introduction to the General Cytology of the Cruciferae

Cytological observations have been made on 155 entities (excluding sterile hybrids) referable provisionally to ca. 140 existing taxa in the fern flora of Ceylon. These have been supplemented by observations on 69 entities (involving 46 additional species) from wild material of ferns from other countries, chiefly Malaya, and on 15 additional species of unknown wild origin growing at Kew. The cytological findings are assembled in relation to the taxonomic system of Copeland (1947) which is discussed in a preliminary way together with the relevant portions of the alternative system of Holttum (1947).

Observations on the cytology and taxonomy of the pteridophyte flora of Ceylon

The observations to be recorded here have been carried out primarily for the purpose of clarifying the taxonomic position and naming of the small organism previously known as Chromulina pusilla Butcher. This had been shown (Manton, 1959a) to possess the pigments and fine structure appropriate to a position in or near the Chlorophyceae but quite inappropriate to a position within the Chrysophyceae to which the genus Chromulina properly belongs. Before selecting a new generic name, it was highly desirable to make some comparative electron microscopical observations on authentically named material of appropriate kinds since the fallibility of the light microscope for study of organisms of this order of size (1–3µ) had been all too clearly exposed. A search was therefore made for a representative or representatives of the only genus of green flagellates known to us from the literature to possess a single posteriorly directed flagellum, namely Pedinomonas.

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Further observations on small green flagellates with special reference to possible relatives of Chromulina pusilla Butcher

The three new species to be described below are the first of a considerable series of related forms, recently isolated from marine plankton off the British coasts and possessing in common a number of rather unusual characters which make their classification difficult. They are all very small and extremely fragile unicellular flagellates, with the characteristic golden brown plastid colour and metabolic products found in the Chrysophyceae to which they undoubtedly belong. Within this group their placing is more difficult and it cannot even be adequately discussed until the whole range of new forms is available for comparison. The most that we can do on this occasion is to make some provisional generic comparisons in order to establish a system of nomenclature. Unfortunately, existing descriptions of relevant genera are both scanty and incomplete, a defect which is easily explained by the nature of the organisms and the need for special facilities for their adequate study.

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Irene Manton

Papers

Problems of cytology and evolution in the Pteridophyta

Introduction to the General Cytology of the Cruciferae

Observations on the cytology and taxonomy of the pteridophyte flora of Ceylon

Further observations on small green flagellates with special reference to possible relatives of Chromulina pusilla Butcher

Studies on marine flagellates II. Three new species of Chrysochromulina