Hidden genomic evolution in a morphospecies—The landscape of rapidly evolving genes in Tetrahymena
Fig 9
Evolutionary model of the observed innovation in LRR genes with tandem 90-bp exons.
(a) Diagram of a Tetrahymena cell. (b) Key to LRR gene-related symbols. (c) Typical MIC chromosome showing the biased distribution of key genetic elements. Central green circle: centromere (not yet fully assembled and characterized). Red and blue shading: biased chromosomal distribution of youngest and most conserved genes, respectively. Darkest color: highest concentration. Pink dashed line above the chromosome: biased chromosomal distribution of TEs, REP included, and other repeated sequences. (d) Multiple exon-shuffling mechanisms proposed to explain how pericentromeric and subtelomeric regions of the MIC genome function as LRR gene innovation centers. (1) Unequal crossing over between 2 different exons of the same LRR gene leads to alleles with more and fewer tandem repeats. (2) Unequal crossing over between exons in 2 different LRR genes leads to exon duplications and deletions. (3) REP retrotransposition into a preexisting LRR gene (step 1), followed by possible (not yet demonstrated) REP-mediated retrotransduction of LRR gene repeats into another LRR gene (step 2) would lead to a net increase in number of LRR gene repeats. Pink line: transcript resulting from cotranscription of REP and 1 LRR gene repeat. Note that the right branch of represents a co-retrotransposition of REP and an LRR repeat, which lead to dispersal of the LRR repeats and could potentially mediate further ectopic recombinations. (4) REP copies, also being repeated sequences, can undergo unequal crossing over, with similar consequences as mechanism 2. (e) Representative product of the above mechanisms: LRR gene with long tandem arrays of 90-bp exons. LRR, leucine-rich repeat; MAC, macronucleus; MIC, micronucleus; non-LTR, Non-long terminal repeat; REP, REP-type retrotransposon; TE, transposable element.