Molecular Data Shine Light on Dark Taxa of Megaselia Flies
By Melissa Mayer
Believe it or not, between an estimated 80 and 90 percent of all species on Earth—including a staggering 4.5 million insect species—remain undescribed. Many of these are “dark taxa,” species-rich groups that may be impossible to tease apart using morphology alone.
In a study published last week in Insect Systematics and Diversity, a research team from the Natural History Museum of Los Angeles County and the Museum für Naturkunde in Berlin looked at the “dark” taxonomy of the fly genus Megaselia. These phorid flies include the species Megaselia sulphurizona, sometimes dubbed “white belts” and which, as the team found, actually includes 15 similar species, described in the paper using mostly molecular data.
Barcoding Dark Taxa
“Dark taxa are incredibly species-rich groups for which there is very little information and that are difficult to study,” says Brian Brown, Ph.D., curator at the Natural History Museum of Los Angeles County and lead author on the study. “There are other names for dark taxa, such as open-ended taxa or just hopeless cases.”
In a separate paper, coauthor Emily Hartop, Ph.D., of the Museum für Naturkunde defined dark taxa as clades containing more than 1,000 species of which fewer than 10 percent have been described. Traditional keys that use couplets to examine morphological characters are insufficient for dark taxa. Brown says there’s a key for British Megaselia, which comprise about 250 species—but it’s so complicated that someone could work through 230 couplets without being certain the outcome is correct. And other geographical clusters contain even more species. At one site where Brown works on Megaselia in Costa Rica, they’ve found 900 species so far.
“I could spend a year trying to develop those structural characters—but nobody’s going to ever use that,” says Brown. “Because it’s just too darn difficult. Like microdissections of the male genitalia. Do you know how small those male genitalia are? Just insanely small.”
Instead, researchers rely on barcoding using a snippet of mitochondrial DNA, known as cytochrome c oxidase subunit I (COI), that varies by animal species. Researchers can use this as a starting point to figure out which specimens are truly separate species and which simply have more variable sequences.
Brown—who has described some 700 species so far—says sometimes that’s straightforward, but, in the case of the white-belted Megaselia, not so much. He calls them a worst-case scenario.
The Megaselia Complex is … Complex
“Megaselia is a genus that I’ve been avoiding studiously my whole career because they’re a hopeless group,” Brown says. “The reason I started working on this group of Megaselia is basically because we started doing inventory. And when you do inventory—instead of doing revisions of groups—then you’re stuck looking at everything.”
Megaselia likely comprises thousands of species all over the world. In Los Angeles, it includes those white belts, so named due to the yellowish dorsal plates of the males. It turns out that fresh specimens have white plates that yellow as they air-dry—and they vary a lot.
The research team began looking at a few white-belted specimens that showed up during the BioSCAN project when they set up malaise traps in backyards and gardens all over Los Angeles.
“When I started looking at the specimens that were pictured on the Barcode of Life database website, I saw that there were more than one Megaselia sulphurizona—I mean species or specimens with widely divergent DNA barcodes,” Brown says.
In the paper, the team describe the species that make up the complex so far. Some of these have differences in wing venation, but delimiting the species required molecular and geographic data.
“I think the most important fact—the most distressing fact—is that we don’t know anything about what they do,” Brown says. “All we know about their life histories is they sometimes fly into malaise traps. So that’s something I’m continuing to work on.”
Brown is also working on the Megaselia species from tropical ecologist Daniel Janzen‘s study in Costa Rica. “I always had a dream of being able to do all of Costa Rica’s phorid flies—put names on all of them,” Brown says. “That dream was completely impossible until barcodes came along. It’s probably [still] impossible, but it’s more likely that I’ll be able to do a big chunk of it.”
By cataloguing the biodiversity of phorid flies, Brown and colleagues hope to better understand the flies’ biologies and the roles they play in their ecosystems—especially those with intense species turnover or where there are endemic species to conserve.
Insect Systematics and Diversity
Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: email@example.com.