Specificity of expression of maternally expressed genes correlates positively with the amount of maternal nutritional contribution. (a) Schematic summarizing the size of transcriptional regulatory regions of maternal and zygotic genes in each species, relative to one another and to the genome-wide average. We note a dichotomy that matches the reproductive mode. The highly conserved relationship between core and metazoan genes' relative 5' IGR regulatory region size suggests that regulatory region length may be considered a metric for complexity and specificity of transcriptional regulation. (b) Organizing animals by the amount of nutritional contribution provided by the mother (low, medium, high), we estimate the specificity of maternal gene expression by the ratio of maternal metazoan gene 5' IGR length to the genome average. Shown are three measures of the ratio of maternal to genome-wide regulatory region lengths for strict-maternal genes (for G. gallus and H. sapiens all maternally expressed genes). Comparison is restricted to metazoan genes, as they comprise the subset most reflective of changes in regulatory complexity.