Mammalian herbivores profoundly influence plant-dwelling insects [1]. Most studies have focused on the indirect effect of herbivory on insect populations via damage to the host plant [2,3]. Many insects, however, are in danger of being inadvertently ingested during herbivore feeding. Here, we report that pea aphids (Acyrthosiphon pisum) are able to sense the elevated heat and humidity of the breath of an approaching herbivore and thus salvage most of the colony by simultaneously dropping off the plant in large numbers immediately before the plant is eaten. Dropping entails the risk of losing the host plant and becoming desiccated or preyed upon on the ground [4,5], yet pea aphids may sporadically drop when threatened by insect enemies [6]. The immediate mass dropping, however, is an adaptation to the potential destructive impact of mammalian herbivory on the entire aphid colony. The combination of heat and humidity serves as a reliable cue to impending mammalian herbivory, enabling the aphids to avoid unnecessary dropping. No defensive behavior against incidental predation by herbivores has ever been demonstrated. The pea aphids' highly adaptive escape behavior uniquely demonstrates the strength of the selective pressure large mammalian herbivores impose on insect herbivores.

Meiosis is a highly conserved process in which a diploid genome is recombined and assorted into haploid gametes. Remarkably, the pea aphid Acyrthosiphon pisum exhibits a reproductive polyphenism whereby environmental signals trigger a switch between apomixis (parthenogenetic reproduction) and meiosis (sexual reproduction). Aphid apomixis results in daughter embryo clones with 2n genome content without male contribution or recombination. This important adaptation allows aphid populations to not only rapidly expand upon abundant resources during summer but also survive winter. How aphids have evolved this ability to switch between parthenogenesis and sexual meiosis is unknown. To arrive at a mechanistic explanation for this developmental plasticity, I determined meiosis gene activity in sexuals and asexuals. I first identified homologs of a core set of meiosis genes from the pea aphid genome. Next, I tested the expression of these core meiosis genes by PCR spanning across at least one intron from cDNA isolated from asexual and sexual ovaries. Surprisingly, meiosis specific genes (e.g., Spo11, Msh4, Msh5, Hop2 and Mnd1) are expressed in asexual ovaries. Additionally, the Spo11 PCR product contained intronic sequence, thus representing unspliced mRNA. Future experiments looking at the quantities and localizations of mRNA and protein will help to distinguish among several possible explanations for these results. Further molecular characterization of this phenotypic plasticity will be helpful in understanding how multiple interacting pathways can evolve to create alternate developmental phenotypes.

Is genetic evolution predictable? Evolutionary developmental biologists have argued that, at least for morphological traits, the answer is a resounding yes. Most mutations causing morphological variation are expected to reside in the cis-regulatory, rather than the coding, regions of developmental genes. This "cis-regulatory hypothesis" has recently come under attack. In this review, we first describe and critique the arguments that have been proposed in support of the cis-regulatory hypothesis. We then test the empirical support for the cis-regulatory hypothesis with a comprehensive survey of mutations responsible for phenotypic evolution in multicellular organisms. Cis-regulatory mutations currently represent approximately 22% of 331 identified genetic changes although the number of cis-regulatory changes published annually is rapidly increasing. Above the species level, cis-regulatory mutations altering morphology are more common than coding changes. Also, above the species level cis-regulatory mutations predominate for genes not involved in terminal differentiation. These patterns imply that the simple question "Do coding or cis-regulatory mutations cause more phenotypic evolution?" hides more interesting phenomena. Evolution in different kinds of populations and over different durations may result in selection of different kinds of mutations. Predicting the genetic basis of evolution requires a comprehensive synthesis of molecular developmental biology and population genetics.

Do general principles govern the genetic causes of phenotypic evolution? One promising idea is that mutations in cis-regulatory regions play a predominant role in phenotypic evolution because they can alter gene activity without causing pleiotropic effects. Recent evidence that revealed the genetic basis of pigmentation pattern evolution in Drosophila santomea supports this notion. Multiple mutations that disrupt an abdominal enhancer of the pleiotropic gene tan partly explain the reduced pigmentation observed in this species.