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Cockroach Genome Harbors Gene Expansions That Could Help it Adapt to Urban Environments

NEW YORK (GenomeWeb) – Researchers have sequenced the 3.4-gigabase genome of the American cockroach, finding it rife with expansions in gene families associated with adapting to various environments.

Cockroaches like Periplaneta americana are pests that live in close proximity with humans and can be disease vectors and trigger allergic reactions and asthma. As a new study reported today in Nature Communications, in addition to gene expansions, the team also found a high level of sequence identity between cockroaches and termites.

"The harm of American cockroaches is becoming more serious with the threat of global warming," the researchers led by the Chinese Academy of Sciences' Shuai Zhan wrote in their paper. "Our study may shed light on both controlling and making use of this insect."

Zhan and his colleagues sequenced three members of the Periplaneta genus — the American cockroach, the Australian cockroach, and the smokybrown cockroach — on the Illumina platform, generating more than a terabyte of data. P. americana was sequenced to 295X coverage, while the Australian cockroach and the smokybrown cockroach were sequenced to about 40X coverage for comparison.

After assembly, the researchers found the American cockroach genome to be 3.38 gigabases large. It is, they noted, the second largest sequenced insect genome, behind that of the locust, and 60 percent of it is made up of repetitive elements. Still, they reported that it has 21,336 protein-coding genes, 95 percent of which are expressed.

When they compared the P. americana genome to a dozen other sequenced insect genomes, including other species within the order Blattodea, the researchers uncovered 479 Blattodea-specific gene orthologs. They also used those genomes to build a phylogeny that indicated Blattodea is monophyletic and that the American cockroach is more closely related to two termite species — the dampwood termite Zootermopsis nevadensis and the fungus-growing termite Macrotermes natalensis — than it is to the German cockroach, Blattella germanica.

The American cockroach also harbored a number of gene expansions that may enable them to adapt to urban environments. It encodes 154 olfactory receptors —twice as many as other blattodeans — 522 gustatory receptors, and 640 ionotropic glutamate receptors. The added olfactory receptors could help the roaches find food more easily, and the gustatory receptors — many of which appeared to be bitter receptors — could be part of a self-protection system that allows them to tolerate bitter and toxic foods, the researchers noted. The researchers also uncovered gene expansions in families like cytochrome P450 enzymes that could help roaches resist insecticides and survive in extreme environments.

Throughout their lifecycle, cockroaches molt between half a dozen and a dozen times and undergo metamorphosis. Using RNAi to knock out key factors, the researchers found that eliminating Met and Kr-h1, which encode the JH receptor and the downstream JH anti-metamorphic factor, led to early metamorphosis, while knocking down Inr, PI3K, and TOR genes slowed the growth rate of nymphs.

During their nymph stage, cockroaches are also able to easily regrow limbs. Knocking down Dpp and Mad, which have been implicated in wound healing and tissue repair in other species, prohibited the regeneration of missing limbs, indicating that the Dpp pathway is needed for wound healing and repair, the researchers found.

This ease of RNAi knockdown in the insect, meanwhile, suggested to the researchers that the cockroach could be used as a model organism.

"[O]ur genomic and functional analyses in the American cockroach provided insights into its success in the adaptation to urban environments and the biology of developmental plasticity in cockroaches," the researchers wrote.