No Rain, No New Friends

Now that we’ve discussed the rainforest dwelling species, it’s only fair that we move on to the river dwelling species so they don’t feel left out of the conversation. Congopanchax brichardi, Epiplatys chevalieri, and Epiplatys Duboisi are found in the marginal vegetation of small streams and rivers in areas exposed to the sun, which is very different than the canopy-covered rainforest we discussed the other three species living in. We found that the haplotype networks of the first two related more closely than that of the third as they both depicted an unexpectedly high genetic diversity. This blog post will therefore discuss our second pattern: river dwelling species with high genetic diversity. Don’t worry, we’ll come back to E. duboisi in the next one. 

If you look at the respective networks, we see the same visual clue of long branches with high nucleotidic differences that we saw in the rainforest dwelling species as well. However, unlike the rainforest dwelling fish networks, the branches aren’t dominated by a single locality’s cluster. Having long branches plus clean separation by site so much less ambiguous and made identifying the first pattern fairly easy. So thanks, river fish, for giving me more work 😛. So, what could explain having shared COI gene codes in fish living miles apart and the presence of many different COI gene codes in the same location? We thought of two possible explanations before narrowing down on one.  

So, it could be the presence of multiple species in the same locality. This would mean that different lineages are living side by side in the same spot, which would explain the surprising amount of genetic variation and could be justified by the easy claim that Killifish are hard to tell apart. Unfortunately, my mentor is always pushing us to take the harder route, which also means you’re going to be reading this for longer than you maybe meant to. The more complex explanation is that these populations were isolated from each other in the past, evolved along separate paths for a while, and then re-established cross-river connectivity and gene flow. This would mean that the genetic diversity isn't multiple species, it's more like several long-separated family branches that have reunited. That reminds me, I got to meet my oldest living relative on my recent trip to India for the first time, and it made me feel so much more connected to that side of the family regardless of how far apart we are lineage-wise and how long we were living halfway across the world from one another.

This pattern of isolation and connectivity has a name: independent populational bottlenecks. Not a buzzword, I promise. We see this clearly happening in Congopanchax brichardi. During moist climate periods in the Congo River basin, where rivers flowed continuously and were connected with one another, C. brichardi populations could expand their distribution ranges and connect with populations in different rivers. But, just as they got past the awkward introduction stage and began making friends and falling in love, then came the arid climate periods. This disconnected the rivers and fragmented populations across various refugia. They began to evolve separately. You might be thinking—separate evolution is conducive to the origin of new species, isn’t it? Yes, it is, and this process can result in that. However, we believe that because the paleoclimatic nature of the Congo is so unpredictable, with periodic isolation and connectivity, these populations remain within the same species, just an incredibly diverse one. However, more research is required to better explain this genetic diversity, and that’s exactly what my mentor and I are currently working on before beginning to write the paper.