Illustration by Alison Schroeer
There is a classic Three Stooges film in which they play bumbling plumbers trying to repair some leaky pipes, and, of course, everything goes wrong. Patching a leak in one place sends water spraying out elsewhere; tugging on a pipe sends the faucet in another room flying out of the wall. It's classic slapstick. It wasn't intended to be deep (and I didn't watch it for a lesson in science!), but it does hold a message that applies to biology: In complex systems, everything is interconnected, and sometimes in surprising ways. Changes to one genetic module can cause effects to ripple throughout the entire organism.
This shouldn't be surprising. In a relationship called allometry, one expected feature of developing embryos is coordinated growth, with different subsystems communicating with one another constantly, negotiating with one another to establish their proper relative proportions. The allometric rules for an animal define how large a particular organ should be, given a particular body size, and they specify how body parts should scale relative to one another. These rules define your proportions: how long your arms should be for a given height, for instance, and for that same height, how long your legs should be. What's required for this to work is that there be interconnected regulators of growth that affect many tissues.
While it means that growth is coupled across the organism, it also implies that there may be tradeoffs. An embryo does not have an infinite amount of energy or growth factors, and changes in the allometric rules therefore should have consequences. One might imagine that trying to change the scaling of a human being to have longer legs for a given height, for instance, might mean legs consume more energy during growth, at the expense of the other set of limbs, the arms. As with the Stooges, trying to pull one pipe forward might mean another gets sucked back into the wall.
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Since we can't do that kind of experiment on people, though, we have to look for an alternative, and a beautiful one presents itself: scarab beetles of the genus Onthophagus. These animals have intense mating competitions in which the males guard their mates to prevent them from breeding with competitors, and they have evolved extravagant weapons for use in battles between males. They have long horns that they use for blocking tunnels to underground dens and for wrestling with competitors, and different species have followed different strategies. Some have a horn at the front of the head and look rhinocerous-like; others sprout a pair of horns, like those of a triceratops, at the back of the head; still others have a long protuberance from their thorax that can make up as much of 40 percent of their body length. These prominent pieces of body armor represent a substantial investment in development. Are there any costs incurred for building them?
Yes, there are. In onthophagine species with horns on the anterior part of the head, horn size is negatively correlated with the size of the antennae, as if the investment in growth of a cuticular horn deprived nearby structures of resources. Similarly, large horns at the back of the head were associated with reduced eye sizes, and thoracic horns were made at the expense of the wings. As a consequence of the interconnectedness of development, shifts in the proportions of one structure are going to have an effect on other structures.
These tradeoffs have been tested experimentally, too. Onthophagus nigriventris is a species with a prominent thoracic horn. Simmons and Emlen took larvae, at a stage termed "late instar," and cauterized the patch of cells that would generate the horn, thereby producing hornless beetles. Losing that horn meant the cauterized beetles actually grew larger on average than control beetles, by about 25 percent.