Patterns of genome size in the copepoda

Adult somatic nuclear DNA contents are reported for eleven cyclopoid specie s (Megacyclops latipes, Mesocyclops edax, M. longisetus, M. ruttneri, M. le uckarti, M. woutersi, Macrocyclops albidus, Cyclops strenuus, Acanthocyclop s robustus, Diothona oculata, Thermocyclops crassus) and for the harpactico id Tigriopus californicus and range from 0.50 to 4.1 pg DNA per nucleus. Th ese diploid genome sizes are consistent with previously published values fo r four Cyclops species (0.28-1.8 pg DNA per nucleus), but are strikingly sm aller than those reported for marine calanoids (4.32-24.92 pg DNA per nucle us). We discuss three explanations, none of them exclusive of another, to a ccount for the smaller size and range of cyclopoid genome sizes relative to calanoid genome sizes: (1) higher prevalence of chromatin diminution in th e Cyclopoida, (2) phylogenetic structure or older age of the Calanoida rela tive to Cyclopoida and (3) nucleotypic selection that may influence life hi story variation and fitness. Measurements of genome size were made on Feulg en stained, somatic cell nuclei, using scanning microdensitometry which is well suited to the sparse and heterogeneous populations of copepod nuclei. The importance of measuring large numbers of nuclei per specimen, possible sources of variation associated with cytophotometric measurements, and appr opriate use of internal reference standards and stoichiometry of the Feulge n stained nuclei are discussed.