I Evolution of Vertebrate Sex Chromosomes and Genomes- 1 Evidence Indicating that the X and the Y or the Z and the W were Originally an Homologous Pair of Ordinary Chromosomes- 2 Differentiation of the Primitive Sex Elements at the Expense of the Y or W, and the Conservation of the Original X or Z- 3 Polyphyletic Evolution of Vertebrates- 4 Conservation of the Original X and Homology of the X-linked Genes in Placental Mammals- 5 Conservation of the Original Z-Chromosome by Diverse Avian Species and Homology of the Z-linked Genes- II Evolution of Dosage Compensation Mechanism for Sex-linked Genes- 6 The Basic Difference in Constitution between the Mammalian X and the Drosophila X- 7 The Two Different Means of Achieving Dosage Compensation for X-linked Genes Employed by Drosophila and Mammals- 8 Various Consequences of the Dosage Compensation by X-inactivation- 9 The Conservation of the Original X and Dosage Compensation in the Face of X-polysomy- 10 Three Different Consequences of X-autosome Translocation- 11 The Consequences of Y-autosome Translocation and the XO as the Normal Female of Certain Mammalian Species- 12 Apparent Absence of Dosage Compensation for Z-linked Genes of Avian Species- III On So-called Sex Determining Factors and the Act of Sex Determination- 13 Elucidation of So-called Sex-determining Factors- 14 Time and Place of Action of Sex-determining Factors in Ontogeny- Author Index

Sex Chromosomes and Sex-Linked Genes

C:N:P ; evolution of growth rate; %N ; nutrient recycling; %P ; stoichiometry; Abstract Understanding the reciprocal interactions between the evolved characteristics of species and the environment in which each species is embedded is a major priority for evolutionary ecology. Here we use the perspective of ecological stoichiometry to test the hypothesis that natural selection on body growth rate affects consumer body stoichiometry. As body elemental composition (nitro- gen, phosphorus) of consumers influences nutrient cycling and trophic dynamics in food webs, such differences should also affect biogeochemical processes and trophic dynamics. Consistent with the growth rate hypothesis, body growth rate and phosphorus content of individuals of the Daphnia pulex species complex were lower in Wisconsin compared to Alaska, where the brevity of the growing season places a premium on growth rate. Consistent with stoichiometric theory, we also show that, relative to animals sampled in Wisconsin, animals sampled in Alaska were poor recyclers of P and suffered greater declines in growth when fed low-quality, P-deficient food. These results highlight the importance of evolutionary context in establishing the reciprocal relationships between single species and ecosystem processes such as trophic dynamics and consumer-driven nutrient recycling.

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The evolution of ecosystem processes: growth rate and elemental stoichiometry of a key herbivore in temperate and arctic habitats

Cytosine methylation is widely distributed in multicellular organisms. We present a comprehensive survey of the existing data on the phylogenetic distribution of DNA methylation in invertebrates, together with new data for the crustacean Penaeus semisulcatus, the annelid Aporrectodea caliginosa trapezoides, and the parasitic platyhelminth Schistosoma mansoni. Two alternative hypotheses addressing the function of cytosine methylation in invertebrates are evaluated: (1) cytosine methylation is an ancient regulatory mechanism which was lost in species with low rates of cell turnover, and (2) cytosine methylation is primarily a defense mechanism against genomic parasites and is expected to be present in all species with large genomes. We discuss the role of DNA methylation in the evolution of development in light of these hypotheses and conclude that gene control and cell memory are important and primitive functions of DNA methylation.

The Role of DNA Methylation in Invertebrates: Developmental Regulation or Genome Defense?

Ploidy-level variation among asexual clones of Daphnia pulex showed a clear latitudinal pattern. Only a single diploid clone was identified at the high-arctic Igloolik site, and it comprised only 2% of the individuals in the populations at that locality (clone 43 in Weider et al. 1987). Conversely, at Windsor, the temperate site, only diploids were found; and at the low-arctic locality of Churchill, both diploids and polyploids were common. The clear association between latitude and ploidy level supports the view that polyploidy is selected for in extreme environments. If representative of a general trend in animals, these results suggest that geographical patterns in the distribution of parthenogens arise from indirect rather than direct selection for shifts in the breeding system.

Geographical Parthenogenesis and Polyploidy in Daphnia pulex

Summary Present knowledge about chromosomes and nuclear DNA of Crustacea is reviewed. Haploid chromosome numbers range from 3 (Acanthocyclops) to 188 (Astacus). Chromosomes are generally small to medium in size (1–5 μm) and punctiform or rod-shape. Nuclear DNA amounts show a large range, lying between 0.37 (Daphnia) and 22.6 pg (Decapoda) per haploid genome, 5.5 × 108 nt to 1.8 × 1010 nt. Highly repetitive sequences may represent as much as 30% of the genome and show a high degree of conservation in Brachyura, while some intermediate repetitive sequences are under-represented. The relationship between taxonomy and cytogenetics appears very complex in some taxa, such as Artemia (Branchiopoda). This genus includes bisexual sibling species with female heterogamety and parthenogenetic populations with different levels of ploidy, up to triploid and pentaploid. Daphnia presents diploid and polyploid strains, and parthenogenetic reproduction. Chromatin diminution occurs in Copepoda; in this group male and female...

Chromosomes and nuclear dna of crustacea

Nuclear DNA amounts have been determined for 42 species of crustaceans bringing the total number of species with known nuclear DNA content to over 70. Genome size in Crustacea varies over a 25-fold range with a modal value of 2 to 3 pg haploid being common in many groups. Both average genome size and the amount of variability among species are characteristic for certain groups. A trend towards small genomes is evident in advanced and specialized crustacean groups. Somatic polyploidy is a very pronounced feature of the Crustacea. The data suggest that evolution by polyploidy may be more common in crustaceans than earlier data had indicated. These features and the presence of very characteristic satellite fractions in the nuclear DNA recommend the Crustacea for further studies in evolutionary genetics.

E. L. Rheinsmith

Papers

Nuclear DNA amounts in pacific Crustacea.