Silks play a crucial role in the survival and reproduction of many insects. Labial glands, Malpighian tubules, and a variety of dermal glands have evolved to produce these silks. The glands synthesize silk proteins, which become semicrystalline when formed into fibers. Although each silk contains one dominant crystalline structure, the range of molecular structures that can form silk fibers is greater than any other structural protein group. On the basis of silk gland type, silk protein molecular structure, and the phylogenetic relationship of silk-producing species, we grouped insect silks into 23 distinct categories, each likely to represent an independent evolutionary event. Despite having diverse functions and fundamentally different protein structures, these silks typically have high levels of protein crystallinity and similar amino acid compositions. The substantial crystalline content confers extraordinary mechanical properties and stability to silk and appears to be required for production of fine protein fibers.

The factors that control replication rate of the intracellular bacteriumWolbachia pipientisin its insect hosts are unknown and difficult to explore, given the complex interaction of symbiont and host genotypes. Using a strain ofWolbachia that is known to over-replicate and shorten the lifespan of itsDrosophila melanogasterhost, we have tracked the evolution of replication control in both somatic and reproductive tissues in a novel host/Wolbachiaassociation. After transinfection (the transfer of aWolbachiastrain into a different species) of the over-replicatingWolbachia popcornstrain fromD. melanogastertoDrosophila simulans, we demonstrated that initial high densities in the ovaries were in excess of what was required for perfect maternal transmission, and were likely causing reductions in reproductive fitness. Both densities and fitness costs associated with ovary infection rapidly declined in the generations after transinfection. The early death effect inD. simulansattenuated only slightly and was comparable to that induced inD. melanogaster. This study reveals a strong host involvement inWolbachiareplication rates, the independence of density control responses in different tissues, and the strength of natural selection acting on reproductive fitness.