Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [22]
Aside from the kind of Trivial Pursuit cum Guinness Book of World Records appeal of this kind of information (though, alas, clues about genome sizes are unlikely to come up in crossword puzzles), what does the variation in genome size—and its lack of relationship to the complexity of the organism in which it resides—mean? Obviously, more isn't better. Bluntly put by Ryan Gregory, a biologist at the University of Guelph and one of the world's leading genome size researchers, this decoupling of DNA content and complexity puts paid "the expectation that genomes consist of the genes, all the genes, and nothing but the genes."
So if the genome contains material other than genes, how did that happen? Furthermore, what exactly is that other material, and what is it doing in there? And why do some organisms seem to have so much more of it than others?
The answers to these questions are intertwined. Some of the "extra" material consists of free-floating bits of DNA, sometimes called transposable elements or, more colorfully, selfish DNA. These arise when a sequence of DNA copies itself several times and then just lingers as part of the genome. It is selfish because, a la Richard Dawkins's selfish gene, the elements benefit by making more copies of themselves, but they do not contribute to the functioning of the organism in which they reside. If there is no disadvantage to the organism of harboring them, or even if there is a cost but no means of getting rid of them exists, they will persist, cluttering up the genome and giving us those oddball genome sizes in some species.
Other noncoding DNA is often called junk DNA, which sometimes is used to mean all types of genetic material aside from the genes themselves, but more properly refers to copies of genes that used to be functional but are now obsolete. Like a manual lawnmower with a broken blade that you tuck away in the garage even after you've bought an electric model, the junk DNA clutters up the genome. In a distinction reminiscent of couples squabbling over organizing the closets, some scientists call DNA junk if it's not functional at the moment but could be useful at some hypothetical time, like that lawnmower, but garbage if it's not functional now and never will be, like—well, maybe it's best not to offer an example here. As with the transposable elements, junk DNA is thought to accumulate because DNA has an inherent tendency to copy itself unless otherwise halted.
Genome size is often, though not always, a reflection of body size, particularly among insects and other invertebrates. And insects that take longer to develop from eggs into adults have larger genomes as well. Another restriction on insect genome size seems to be the way that the species develops—does it go through a metamorphosis with egg, caterpillar, cocoon, and adult stages, like a butterfly, or does each successive stage look like a slightly pumped-up version of the one before, like a grasshopper? The butterfly types seem to have far smaller genomes than the grasshoppers, for reasons that are unknown. Also perplexing is a link between sperm length, which as I discuss further in a later chapter varies enormously among insects, and genome size. And intriguingly, all insects that exhibit social behavior, including not just bees and wasps but termites, as well as cockroaches that take care of their young after hatching, have reduced genomes, despite the vast evolutionary distance between these groups.
I look forward to the solutions to questions about genome variation, but what I like best about the measurements of genome size is the way