WEIDER, LJ; Univ. of Oklahoma: rDNA intergenic spacer (IGS) length variation and its significance for influencing growth and/or developmental rates of organisms.

The ribosomal (r)DNA molecule is a multi-gene family, and in eukaryotes, consists of a transcription unit that includes the genes for 18S, 5.8S, and 28S rRNA subunits (�rrn� genes), the internal transcribed spacers separating the genes within a unit, and an intergenic spacer (IGS) separating adjoining units. rDNA is critical in transcription and translation leading to the production of proteins. Multiple copies of rDNA motifs, ranging from 10s to 1000s of copies per cell, are found across a broad range of taxa. The IGS is the most rapidly evolving region of the rDNA molecule, and often consists of a variety of sub-repeats, with each sub-repeat varying in size from less than 100 base pairs (bp) to several hundred bp in length. Promoter, enhancer, and terminator sites for transcription are known to occur in these sub-repeats in a variety of organisms. Due to mechanisms such as slippage and unequal crossing over during mitosis, multiple IGS length variants can occur within populations, individuals, and even single clones. In this present paper, I will review some of the current literature related to how variation in the number of sub-repeats leads to length heterogeneity within the IGS, and in turn how this length heterogeneity may impact important organismal functions such as developmental and growth rates. I will use a recent example of some work that is looking at the relationship between IGS length variation, stoichiometric relationships (i.e. C:N:P ratios), RNA:DNA ratios, and growth rate in the microcrustacean zooplankter genus, Daphnia, to illustrate how an integrative approach is essential in linking important sub-cellular processes to organismal and population-level processes that can influence the ecology and evolution of taxa.