I mentioned sex combs a while back, so I thought I’d clarify a bit — I hope none of you rushed out to buy one for yourself (I don’t think human sex combs exist, but if they do, I don’t need to know.) Sex combs are secondary sexual characteristics found in on the forelegs of only male Drosophilidae. They are small dark patches of bristles on the tarsus of the first leg, and they are not something you’d notice if you saw a fly buzzing in your kitchen — you have to knock them out and carefully scrutinize the limbs with a hand lens or microscope to see them, but they’re important for recognizing the sex of a fly definitively. I tell my genetics students that you can tell the sexes apart by the shape of the abdomen or the pigment patterns, but to be really sure you should check for the presence or absence of the sex combs.

They don’t look like much, but they also matter to female flies. Mutants or surgically modified male flies with the sex combs reduced suffer with lower reproductive success. The flies use them as gentle grasping tools to separate her wings, grasp her abdomen, and tease open the genitals, so of course they’re subject to selection. Different species of fruit flies exhibit different patterns of sex combs, and we observe natural variation within a species.

Variation within and between species makes sex combs of great interest to evolutionary biologists. Here’s an illustration of the variation we can see.

Focus for now on the top panel on the right, which shows a male and a female foreleg. They both have hairy legs, and in the default pattern seen in the females is a series of bristles in transverse rows (TRs) in arrays marching down the leg. The flies specifically use these bristles to groom their eyes — if you look closely, flies are remarkably tidy and neat.

The TRs are illustrated diagrammatically as small open circles in rows, the base pattern. These bristles are also developmentally interesting, because the way you make a sex comb is you express a set of specific genes in the distal two rows, and the whole structure rotates 90° to form a longitudinal comb. This opens up a whole set of informative interactions — the rotation is essential for function, and is subject to constraints imposed by adjacent tissues. I’ve been reading papers for the past week focused on the developmental and evolutionary significance of this tiny, odd, little known structure in flies. You should read some of these papers, too! I’m a sucker for anything evo-devo, and that’s what this little patch of hairs illustrates.

The most complex and diverse secondary sexual character in Drosophila is the sex comb (SC), an arrangement of modified bristles on the forelegs of a subclade of male fruit flies. We examined SC formation in six representative nonmodel fruit fly species, in an effort to understand how the variation in comb patterning arises. We first compared SC development in two species with relatively small combs, Drosophila takahashii, where the SCs remain approximately transverse, and Drosophila biarmipes, where two rows of SC teeth rotate and move in an anterior direction relative to other bristle landmarks. We then analyzed comb ontogeny in species with prominent extended SCs parallel to the proximodistal axis, including Drosophila ficusphila and species of the montium subgroup. Our study allowed us to identify two general methods of generating longitudinal combs on the tarsus, and we showed that a montium subgroup species (Drosophila nikananu) with a comb convergently similar in size, orientation and position to the model organism Drosophila melanogaster, forms its SC through a different developmental mechanism. We also found that the protein product of the leg patterning gene, dachshund (dac), is strongly reduced in the SC in all species, but not in other bristles. Our results suggest that an apparent constraint on SC position in the adult may be attributable to at least two different lineage-specific developmental processes, although external forces could also play a role.

Atallah J, Liu NH, Dennis P, Hon A, and Larsen EW (2009) Developmental constraints and convergent evolution in Drosophila sex comb formation. Evolution & Development 11(2): 205-218.

Malagón JN, Ahujab A, Sivapatham G, Hung J, Leea J, Muñoz-Gómez SA, Atallah J, Singh RS, and Larsena E (2013) Evolution of Drosophila sex comb length illustrates the inextricable interplay between selection and variation www.pnas.org/cgi/doi/10.1073/pnas.1322342111

I’m not a fan of phys.org — they summarize interesting articles, but it’s too often clear that their writers don’t have a particularly deep understanding of biology. I wonder sometimes if they’re just as bad with physics articles, and I just don’t notice because I’m not a physicist.

Anyway, here’s a summary that raised my hackles.

Chromosphaera perkinsii is a single-celled species discovered in 2017 in marine sediments around Hawaii. The first signs of its presence on Earth have been dated at over a billion years, well before the appearance of the first animals.

A team from the University of Geneva (UNIGE) has observed that this species forms multicellular structures that bear striking similarities to animal embryos. These observations suggest that the genetic programs responsible for embryonic development were already present before the emergence of animal life, or that C. perkinsii evolved independently to develop similar processes. In other words, nature would therefore have possessed the genetic tools to “create eggs” long before it “invented chickens.”

First two words annoyed me: Chromosphaera perkinsii ought to be italicized. Are they incapable of basic typographical formatting? But that’s a minor issue. More annoying is the naive claim that a specific species discovered in 2017 has been around for a billion years. Nope. They later mention that it might have “evolved independently to develop similar processes”, which seems more likely to me, given that they don’t provide any evidence that the pattern of cell division is primitive. It’s still an interesting study, though, you’re just far better off reading the original source than the dumbed down version on phys.org.

All animals develop from a single-celled zygote into a complex multicellular organism through a series of precisely orchestrated processes. Despite the remarkable conservation of early embryogenesis across animals, the evolutionary origins of how and when this process first emerged remain elusive. Here, by combining time-resolved imaging and transcriptomic profiling, we show that single cells of the ichthyosporean Chromosphaera perkinsii—a close relative that diverged from animals about 1 billion years ago—undergo symmetry breaking and develop through cleavage divisions to produce a prolonged multicellular colony with distinct co-existing cell types. Our findings about the autonomous and palintomic developmental program of C. perkinsii hint that such multicellular development either is much older than previously thought or evolved convergently in ichthyosporeans.

Much better. The key points are:

• C. perkinsii is a member of a lineage that diverged from the line that led to animals about a billion years ago. It’s ancient, but it exhibits certain patterns of cell division that resemble those of modern animals.
• Symmetry breaking is a simple but essential precursor to the formation of different cell types. The alternative is equipotential cell division, one that produces two identical cells with equivalent cellular destinies. Making the two daughter cells different from each other other opens the door to greater specialization.
• Palintomic division is another element of that specialization. Many single-celled organisms split in two, and each individual begins independent growth. Palintomic division involves the parent cell undergoing a series of divisions without increasing the total cell volume. They divide to produce a pool of much smaller cells. This is the pattern we see in animal (and plant!) blastulas: big cell dividing multiple times to make a pile of small cells that can differentiate into different tissues.
• Autonomy is also a big deal. They looked at transcriptional activity to see that daughter cells had different patterns of gene activity — some cells adopt an immobile, proliferative state, while others develop flagella and are mobile. This is a step beyond forming a simply colonial organism, is a step on the path to true multicellularity.

Cool. The idea is that this organism suggests that single-celled organisms could have acquired a toolkit to enable the evolution of multicellularity long before their descendants became multicellular.

I have a few reservations. C. perkinsii hasn’t been sitting still — it’s had a billion years to evolve these characteristics. We don’t know if they’re ancestral or not. We don’t get any detailed breakdown of molecular homologies in this paper, so we also don’t know if the mechanisms driving the patterns are shared.

I was also struck by this illustration of the palintomic divisions the organism goes through.

Hang on there : that’s familiar. D’Arcy Wentworth Thompson wrote about the passive formation of cell-like cleavage patterns in simple substrates, like oil drops and soap bubbles, in his book On Growth and Form, over a century ago. You might notice that these non-biological things create patterns just like C. perkinsii.

That does not undermine the paper’s point, though. Multicellularity evolved from natural processes that long preceded the appearance of animals. No miracles required!