Ribosomal 5S genes in relation to C-value in amphibians

We have measured the amount of 5S-ribosomal DNA in the genomes of Xenopus laevis, Triturus cristatus carnifex and Ambystoma mexicanum, three species of Amphibians which have widely different C-values. Our best estimate is that these organisms have about 24,000, 32,000 and 61,000 5S-genes per haploid genome respectively. A trend to increasing 5S gene copynumber with increasing C-values in amphibians is apparent, probably linked to the need to supply more ribosomes to the larger cells which are associated with larger genomes, particularly during the critical phases of oogenesis and embryonic cleavage. The correlation between the two is poor however, and whilst C-value may determine a minimum gene copy-number, there appears to be little constraint on exceeding this minimum in some species. Certain problems encountered in measuring gene copy-numbers, i.e. the criterion dependance of such numbers and the effect of having pseudogenes, are highlighted.