UNUSUALLY HIGH NUMBERS OF RIBOSOMAL-RNA GENES IN COPEPODS (ARTHROPODA, CRUSTACEA) AND THEIR RELATIONSHIP TO GENOME SIZE

We report on copy numbers of 18S ribosomal RNA genes in three species of copepods (Crustacea: Copepoda), two of which possess an unusual arr angement in which 5S genes are included within the 18S-5.8S-28S repeat unit. Slot blots of genomic and standard DNA were hybridized with an 18S rRNA gene probe constructed from one of the marine species and hyb ridization was quantified using chemiluminescence. Diploid 18S rRNA ge ne copy numbers are estimated as ca. 15 300 and 33 500 in the marine s pecies Calanus finmarchicus (13.0 pg DNA in 2C adult nuclei) and C. gl acialis (24.2 pg DNA), respectively, and ca. 840 and 730 in two freshw ater populations of Mesocyclops edax (both ca. 3 pg DNA) from Virginia and Nova Scotia, respectively. The roughly proportional relationship between 2C somatic nuclear DNA contents and rRNA gene copy number in t he sibling species C. finmarchicus and C. glacialis may reflect polyte nic replication of entire genomes during abrupt speciation events. Cop y numbers may also reflect differential losses during embryonic chroma tin diminution.