KARYOLOGY, NUCLEAR-DNA QUANTIFICATION AND NUCLEUS-CYTOPLASMIC DOMAIN VARIATIONS IN SOME MULTINUCLEATE GREEN-ALGAE (SIPHONOCLADALES, CHLOROPHYTA)

Chromosome complements of 1 N = 12, 24 and 16, 32 are reported for fou r species of Siphonocladales. Spatial arrangement of nuclei through th e division cycle is described. MicrospectrophotometrY with the DNA-loc alizing fluorochrome DAPI is used to estimate nuclear genome sizes in twelve species. Nuclear genome sizes estimated from nuclear volumes (N V) obtained with image analysis measurements were found to be highly c orrelated with estimates from fluorescence intensity (I(f)) values. Nu clear genome sizes in these taxa range from 1. 1 to 19.2 pg, with 2 C nuclei having DNA contents of 2.6-4.9 pg. Homologous haploid and diplo id cells appear to maintain a constant nuclear genome content by regul ating the number and/or ploidy level of nuclei. Interspecific comparis ons suggest an inverse relationship between genome size and 'area of c ytoplasmic domain' per standardized nuclear DNA unit. The possibility is discussed that the very large nuclear genomes in these algae may ha ve two functions in addition to a genic role: (1) maintaining NV:cytop lasm ratios in large coenocytic cells and 2 balancing increased extra- nuclear plastid DNA concentrations.