Chromosomal evolution in sandpipers

Dynamic reshuffling of chromosomes across the Charadriiformes phylogeny.

In general, birds have a very stable number of chromosomes. Most species house 40 pairs of chromosomes – so, 80 in total – in their cells (commonly noted down as 2n = 80). These chromosomes can be divided into a few huge macrochromosomes and several tiny microchromosomes. However, not all species adhere to this “2n = 80 rule” in birds. In biology, there are always some exceptions. Falcons, for example, show chromosomal numbers of only 2n = 40 to 2n = 52. These atypical counts are probably the outcome of fusions between microchromosomes into macrochromosomes (see this blog post for more about the peculiar genomes of falcons). The order Charadriiformes – waders, gulls and allies – exhibit a wide range of karyotypes, ranging from 2n = 42 in the Eurasian Thick-knee (Burhinus oedicnemus) to 2n = 98 in the Common Snipe (Gallinago gallinago). To gain more insights into the patterns of chromosomal evolution in this bird group, researchers took a closer look at the karyotype of the Spotted Sandpiper (Actitis macularius). Their findings appeared in the journal BMC Ecology and Evolution.

Fissions

The Spotted Sandpiper has 92 chromosomes of which 14 pairs are macrochromosomes. To reconstruct the chromosomal evolution of this species, the researchers “painted” the chromosomes of the Spotted Sandpiper onto the karyotype of the Eurasian Thick-knee. This molecular technique revealed how different chromosomes were rearranged in the two species. It turns out that in the Spotted Sandpiper several chromosomes have been split into two or more smaller chromosomes. For instance, the second chromosome pair in the Eurasian Thick-knee (denoted as BOE2) split into four new pairs in the Spotted Sandpaper (namely AMA3, AMA11, AMA12 and AMA13). Similarly, BOE3 was divided into AMA4, AMA14 and AMA15. The figure below provides a nice overview of this karyotypic puzzle.

An overview of the chromosomal rearrangements between the Eurasian Thick-knee (BOE) and the Spotted Sandpiper (AMA). From: Pinheiro et al. (2021).

Phylogeny

By combining the chromosomal information of the Eurasian Thick-knee and the Spotted Sandpiper with karyotypic knowledge of other bird species, the researchers managed to reconstruct the evolutionary history of chromosome numbers in the order Charadriiformes. Most fissions occurred quite early in the evolution of these birds, namely after the gulls (family Laridae) split from the other families. An additional fission probably took place at the base of the sandpiper family Scolopacidae. Interestingly, rearrangements in the opposite direction – fusion of several chromosomes – happened within the Jacanidae family, giving rise to an ancestral-like karyotype of 2n = 82.

Seeing all these chromosomal reshuffling, I cannot help but wonder whether these changes were adaptive. Did the birds benefit from having more or less chromosomes in their cells? Or are these just non-adaptive rearrangements without much impact on individual fitness? Exciting questions that will hopefully be addressed in a future blog post.

Chromosomal changes across the phylogeny of the Charadriiformes. The karyotypes at the bottom depict the putative ancestral karyotype (PAK) of birds and the PAK of Scolopaci (SPAK).

References

Pinheiro, M. L. S., Nagamachi, C. Y., Ribas, T. F. A., Diniz, C. G., Ferguson-Smith, M. A., Yang, F., & Pieczarka, J. C. (2021). Chromosomal painting of the sandpiper (Actitis macularius) detects several fissions for the Scolopacidae family (Charadriiformes). BMC Ecology and Evolution, 21(1), 1-10.

Featured image: Spotted Sandpiper (Actitis macularius) © Mike Baird | Wikimedia Commons