Search for pests by DNA

The starfish destroying coral reefs was found with the help of test strips

Alice Bakhareva, N+1

Australian scientists have proposed using immunochromatographic test strips to determine the DNA of acanthaster starfish in seawater and monitor their abundance.

Acanthasters feed on corals and destroy large areas of coral reefs during outbreaks; constant monitoring of populations helps to prevent such outbreaks. The test strips turned out to be a more sensitive, cheaper and faster method compared to other DNA tests and will be able to replace the time-consuming methods of direct observation that are used now. The article was published in the journal Environmental DNA (Jason Doyle & Sven Uthicke, Sensitive environmental DNA detection via lateral flow assay (dipstick)—A case study on corallivorous crown‐of‐thorns sea star (Acanthaster cf. solaris) detection).

The crown of thorns, or acanthaster (Acanthaster planci) is a starfish that feeds on corral polyps. One animal eats up to 13 square meters of coral per year, and due to regular outbreaks of their numbers, the areas of coral reefs, including the Great Barrier Reef, are significantly reduced.  In order to protect the reefs, scientists constantly monitor populations of crown of thorns, stop and, if possible, prevent outbreaks of their numbers. Usually acanthasters are considered divers who are towed over a section of the reef. This method is laborious, not too accurate and allows you to notice only 10-20 percent of thorn crowns.  Jason Doyle and Sven Uthicke from the Australian Institute of Marine Sciences have developed a simple and accurate way to determine the presence of crowns of thorns. They detected the DNA of these starfish in water samples using immunochromatographic analysis, or test strips - pregnancy or HIV tests work on the same principle.  Water samples from two points of the Great Barrier Reef were filtered onto a membrane, then a polymerase chain reaction was carried out to amplify the signal: the enzyme repeatedly copied DNA sections that corresponded to a sample — a primer - specific for acanthasters. At the ends of the obtained DNA copies there were tags with which they were detected by a test strip.  The multiplied samples with labels were transferred to the edge of the immunochromatographic strips, with the help of capillary forces, the liquid moved up the strip. In the incubation zone, the label at one end of the DNA copies was bound to a carbon nanoparticle, then in the test zone, the second label was bound to antibodies, and pieces of DNA were fixed in a certain section of the strip. It turned out to be a "sandwich": antibodies on the strip, starfish DNA and a carbon nanoparticle, are such "sandwiches" and form a transverse line on the test strip. The second line was a control line, it bound free nanoparticles and by its manifestation it was possible to judge that the test was successful and that the sample fell into the test zone.

The effectiveness of the test was first tested on samples with a known concentration of acanthaster DNA, for this purpose samples were isolated from aquarium thorn crowns. The sensitivity of the test strips was higher than that of the other two PCR assays (with detection in agarose gel and digital drip PCR): eight out of eight tests detected acanthaster DNA in an image with seven copies of the molecule (0.11 picograms of DNA), and seven out of eight - in a sample with two copies (0.036 picograms of DNA). The test strips also turned out to be cheaper than digital drip PCR, whose sensitivity was almost as high: one immunochromatographic test costs about eight Australian dollars, and digital PCR costs about 16, while the first test takes three minutes, and the second takes seven.  The DNA of the crown of thorns was found in 26 of the 40 water samples taken from Lizard Island, and only in one sample from Elizabeth Reef. Divers who estimated the number of acanthasters at the same time found few starfish on Lizard Island and none on Elizabeth Reef. The immunochromatographic analysis data thus reflected the number of starfish measured by direct observation: with a small number of acanthatsters, their DNA was found in half of the samples, and an increase in the proportion of positive samples would indicate an increase in the number of echinoderms. This means that monitoring can be carried out by this simple and cheap laboratory method: it will detect the appearance of thorn crowns on the reef and increase their population and prevent an outbreak of numbers in time.  Ecologists have long used DNA from the environment to estimate the abundance of a wide variety of aquatic species: for example, scientists can use samples of seawater to determine the population of whale sharks and its abundance. But no one has used test strips to analyze such samples yet - doctors use them more often to determine various viruses, hormones and even the level of vitamins in the blood.

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