5' IGR length in all animals is dependent on both gene functional complexity and transcript origin of synthesis. (a) Genetic manipulation of D. melanogaster enables quantification of the maternal and zygotic components of mRNA abundance, allowing analysis of five gene classes. Genes expressed solely by the zygote have long 5' IGRs, whereas genes expressed by the mother have short 5' IGRs. Observed differences are greatest when comparing genes expressed exclusively from one origin. (b-d) Similar comparisons for C. elegans, D. rerio and M. musculus, where gene classification is based solely on characteristic strict-maternal and strict-zygotic expression profiles. In mouse an inverse relationship between maternal and zygotic genes is observed. (e,f) 5' IGR length comparison of all maternally expressed genes in G. gallus and H. sapiens to all other genes in the genome. Like mouse, human maternal genes have large 5' IGRs. In all plots, genes were partitioned to core and metazoan classes by phylogenetic filtering. Core genes have shorter 5' IGRs than metazoan ones. Numbers in parentheses to the right of each box plot bar are numbers of genes per class.