The cockroach is one of the insects that everybody wants to kill. It is often found in dark and moist corners of homes, dining establishments, and offices, specifically in toilets and in the kitchen.

Chinese researchers sequenced the DNA of the American cockroach to discover the secret to the insect's ability to thrive in some of the most disgusting places on Earth. To understand how the species – which was introduced to the US from Africa in the early 16th century – succeeded in filthy and unsanitary corners, scientists deciphered the genetic makeup of the Periplaneta Americana, or the American cockroach, The Guardian reported.

Over 20,000 genes

Shuai Zhan, a researcher at the Institute of Plant Physiology and Ecology in Shanghai, China, discovered that the American cockroach has more than 20,000 genes which make its genetic code as large as that of humans. An unusually large group of genes apparently helps the cockroach survive in filthy places it has adapted to.

The expanded set of genes help the cockroach sense the smell that comes from food, particularly fermented food which the insect likes the most. It has another group of genes that make up the internal detoxification of the insect that protect the cockroach if it eats anything toxic.

There is another set of genes that boost the immune system to fight infection from the germs the bug encounters. All these genes make the cockroach more resilient despite the filth of its environment.

Another set of genes ensure that the cockroach reproduces at an impressive rate, while there are genes that explain the ability of the insect to regrow any limbs that are broken or eaten by predators.

Shuai said that by identifying the genes that are key to the cockroach's survival, researchers hope to find ways to better control the bug's population. The study was published on Tuesday in Nature Communications journal.

He added that their findings help people fight the growing pest that spreads disease, causes allergies, and turns homes into a stinky mess. But there are actually only 30 out of the 4,500 species that actually live near people, Shuai said.

He stressed that the harm of the American cockroach is becoming more serious about the threat of global warming. The study shed light on controlling and making use of the species.

Comparisons to related species

Besides sequencing the genes of the American cockroach, the researchers also compared it to the genes of two related species, Gizmodo reported. These are the Australian cockroach (Periplaneta australesiae) and smokybrown cockroach (Periplaneta fuliginosa). The researchers also compared the American cockroach to genetic data from other insects that belong to the same order of Blattodea. These include the termite and the German cockroach (Blatella germanica).

Genomeweb noted that the Periplaneta Americana was sequenced to 295 times coverage, but the Australian cockroach and the smokybrown cockroaches were sequenced to about 40 times coverage for comparison. They found that the American cockroach's genome to be 3.38 gigabases large. But they observed that 60 percent of its genome is made up of repetitive elements. The researchers found that the American cockroach has 21,336 protein-coding genes, 95 percent of which are expressed.

The researchers also uncovered 479 Blattodea-specific gene orthologs. They then used the genomes to build a phylogeny which indicated that the Blattodea is monophyletic.

They discovered that the American cockroach has one of the largest genomes known to exist among insects. It was second to the common locust. But the species was genetically very close to two Periplaneta cockroaches. When the researchers compared its genes they found that it was more related to two species of termites than to the German cockroach. These are the dampwood termite (Zootermopsis nevadensis) and the fungus-growing termite (Macrotermes natalensis).

The researchers said that the discovery means that American cockroaches could serve as a valuable model to study the evolutionary relationships between termites and cockroaches. They also found evidence that certain families of genes of the cockroach have become more expansive over time, specifically those associated with detoxification and chemoreception.

Chemoreception refers to the ability of the insect to smell and taste the world around it. The researchers noted that the American cockroach has accumulated a very large stockpile of genes that provided the insect with more bitter taste receptors. It likely has helped the species expand across the world by letting the cockroach stomach a wide variety of food wherever the insect ends up.

The detoxification genes help the cockroach survive and evolve resistance to chemical weapons that humans have developed against the insect. The gene expansion in families like the cytochrome P450 enzymes helps the cockroaches resist insecticides and survive in extreme environments.

The American cockroach has a number of gene expansions that enables it to adapt to urban environments. It is because the species encodes 154 olfactory receptors – which is twice as many as other Blattodeans – 522 gustatory receptors, and 650 ionotropic glutamate receptors.

[researchpaper 리서치페이퍼=Vittorio Hernandez 기자]