Fig. 4.
Degree of structural homology between gene paralogs and copy-number change in rRNA genes. (A) Duplication events can yield gene copies with varying degrees of structural resemblance to the ancestral locus depending on the span of duplication and the location of duplication breakpoints. G1 and G2 represent ancestral loci; G1′ and/or G2′ represent duplications of G1 and G2 genes, respectively. Complete gene duplicates were identified as those resulting from a duplication event that spanned, at a minimum, the entire ORF and untranslated regions of one ancestral locus (in this case, G1′ is a complete duplicate of G1). Partial gene duplicates originate when only a portion of the ORF of the ancestral locus is duplicated due to the presence of at least one duplication breakpoint falling within the ancestral ORF (G1′ is a partial duplicate of G1). Chimeric gene duplicates comprise duplicated segments of two protein-coding genes resulting in a single ORF. (B) The frequencies of three structural categories of gene duplicates vary significantly with population size (G = 155.2, P < 2.2 × 10−16). No significant difference is found between the distribution of N = 10 vs. N = 100 MA lines (G = 0.2, P = 0.9). (C) Greater than fourfold variation in rRNA gene copy number in the C. elegans MA lines. There is no relationship between population size and rDNA copy number (F = 1.57, P = 0.22). However, the average copy number of rRNA genes across the MA lines differed significantly from that of the ancestral control (t = 4.96, P = 2.23 × 10−5; red dashed line).