Genome Size Covaries More Positively with Propagule Size than Adult Size: New Insights into an Old Problem

SIMPLE SUMMARY: The amount of hereditary information (DNA) contained in the cell nuclei of larger or more complex organisms is often no greater than that of smaller or simpler organisms. Why this is so is an evolutionary mystery. Here, I show that the amount of DNA per cell nucleus (‘genome size’) relates more positively to egg size than body size in crustaceans (including shrimp, lobsters and crabs). Genome size also seems to relate more to the size of eggs or other gametes and reproductive propagules (e.g., sperm, spores, pollen and seeds) than to adult size in other animals and plants. I explain these patterns as being the result of genome size relating more to cell size (including that of single-celled eggs) than the number of cells in a body. Since most organisms begin life as single cells or propagules with relatively few cells, propagule size may importantly affect or be affected by genome size regardless of body size. Relationships between genome size and body size should thus become weaker as body size (and the amount of cell multiplication required during development) increases, as observed in crustaceans and other kinds of organisms. ABSTRACT: The body size and (or) complexity of organisms is not uniformly related to the amount of genetic material (DNA) contained in each of their cell nuclei (‘genome size’). This surprising mismatch between the physical structure of organisms and their underlying genetic information appears to relate to variable accumulation of repetitive DNA sequences, but why this variation has evolved is little understood. Here, I show that genome size correlates more positively with egg size than adult size in crustaceans. I explain this and comparable patterns observed in other kinds of animals and plants as resulting from genome size relating strongly to cell size in most organisms, which should also apply to single-celled eggs and other reproductive propagules with relatively few cells that are pivotal first steps in their lives. However, since body size results from growth in cell size or number or both, it relates to genome size in diverse ways. Relationships between genome size and body size should be especially weak in large organisms whose size relates more to cell multiplication than to cell enlargement, as is generally observed. The ubiquitous single-cell ‘bottleneck’ of life cycles may affect both genome size and composition, and via both informational (genotypic) and non-informational (nucleotypic) effects, many other properties of multicellular organisms (e.g., rates of growth and metabolism) that have both theoretical and practical significance.