Who’s watching the kids?

by Andrew Anderson (@AndersonEvolve)

I am an evolutionary biologist and, as such, I find the diversity of life to be amazing and love pondering how divergences between species/populations occurred.  But I have a confession: fishes are the best, hands down. Sexual selection and parental behavior are often intertwined, and fishes cover a wide breadth of behaviors, traits, and systems, especially when compared to other vertebrates.  I am sure that there are probably a lot of entomologists and other invertebrate biologists shaking their fists at the screen right now. I’ll concede that those taxa are also amazing in their range of adaptations, but fishes are just cooler, so I’ll focus on them.

Throughout my time on this blog, I hope to point out many different features and adaptations of fishes as well as what processes may have caused them.  I hope to touch on everything from males that look like females to sneak mates, to males who steal eggs from rival nests to make it seem like more females have chosen them, to a species that loses 20% of its genome from its somatic cells (that is, cells that won’t make eggs/sperm).  For this post, though, I’ll touch on a fascinating evolutionary outcome that I really hope to delve into more as my career progresses: which sex watches the kids.

pipepop
Male Gulf pipefish, Syngnathus scovelli, exhibiting male brood care.  He’s ready to pop!

Fishes are unique, because in over half of species that care for their offspring, the male engages in parental behaviors instead of the female.  The lab I entered studies Syngnathids, a.k.a. pipefishes and seahorses, which are known for their male pregnancy. What I found out is the family of fishes that are the closest relatives of Syngnathids, Solenostomids, also have brood care.

ghost
The ghost pipefish on the bottom is a female with larger fins on its underside that form a pouch to hold eggs until they develop into juveniles.  The top fish is male.

You can see some similarities between the two families, but in Solenostomids, a.k.a. ghost pipefish, the females have evolved brood care.  So we have two closely related families that evolved a pouch to hold developing offspring, but the sex responsible is flipped. What caused that?  A good start point would be to figure out what sex cared for offspring in the ancestor to both groups. Without a strong fossil record we can only infer what that ancestor might have looked like by comparing what traits the next closest families to Syngnathids and Solenostomids might have had. It turns out the next families are trumpetfish, cornetfish, and shrimpfish, none of which engages in brood care. As a result, there are three possibilities: 1) the ancestor had male brood care, 2) the ancestor had female brood care, or 3) the ancestor had no brood care and Syngnathids and Solenostomids independently evolved it with a different sex.

sam
Samurai gourami, Sphaerichthys vaillanti, with a secondary sex trait of bands.  Female is on the top with two males below

In order to tease apart what might have happened, I needed to know if there were other groups of fishes that changed which sex took care of the offspring.  Sure enough, I was able to study this using a group of gouramis. Most species of gouramis have male brood care, and in one species, the samurai gourami, females evolved a secondary sex trait (a trait that is different between the sexes that isn’t directly involved in reproduction that may be used to attract or compete over mates).  

choc
Chocolate gourami, Sphaerichthys osphromenoides, whose sex is not determined.

The closest relative to samurai gourami is the chocolate gourami, which has female brood care and is monomorphic between the sexes. I have sequenced the genomes of both gourami species and I am working on acquiring the transcriptome (what genes are turned on and how many times is a particular gene activated). My hope is to piece together what happened at the genomic level to cause such a wholesale behavioral change.

While this work is personal and I’m excited to share it, my goal is to show readers of this page some other peculiar results of evolution, especially in that most extraordinary group: the fishes.  Until next time