Genetic patterns along the South American dry diagonal: The case of the Narrow-billed Woodcreeper

What environmental factors determine the genetic population structure of this species?

South America is known for its lush rainforests and high Andean mountain peaks. Tropical and subtropical rainforests can be found along the Amazon river and on the Atlantic coast. These regions are separated by a broad corridor of open vegetation. This so-called dry diagonal is often seen as a formidable barrier for avian rainforest species that occur on either side of it (a topic I covered in this blog post). For other bird species, however, the dry diagonal is not an obstacle but an important habitat. Indeed, numerous birds live in the several biomes that make up this open vegetation landscape, such as the Chaco, Cerrado and Caatinga.

Given that birds in open environments tend to show high dispersal capacity, we can expect gene flow between neighboring populations. Only when the distances become too large will the levels of gene flow start to decrease. This phenomenon is known as isolation-by-distance, and can easily be tested by correlating genetic divergence with geographical distance. A recent study in the Journal of Avian Biology tested this prediction in the Narrow-billed Woodcreeper (Lepidocolaptes angustirostris), a species that be found throughout the dry diagonal. Will this species show isolation-by-distance or do other factors play a role?

The expected pattern of isolation-by-distance: a positive relationship between genetic divergence and geographical distance. Here illustrated by a landrace of Thai rice. From: Pusadee et al. (2009) PNAS.

Genetic Lineages

Amanda Rocha and her colleagues collected 63 individuals of the Narrow-billed Woodcreeper, covering the main biomes within the open vegetation corridor (Chaco, Cerrado and Caatinga). The sampling effort included representatives from the two main morphological groups within this species: group angustirostris, with brown back and heavily streaked pattern in the chest and vent (containing the subspecies angustirostris, praedatus, certhiolus and hellmayri), and group bivittatus, with more rufescent in the back and unstreaked pattern below (containing the subspecies bivittatus, griseiceps, coronatus and bahiae).

Genetic analyses of the mitochondrial gene ND2 revealed five main lineages: one in Caatinga, three in Cerrado (NE_Cerrado, E_Cerrado and W_Cerrado) and one in Chaco. Hence, each genetic lineages corresponds to a particular biome in the dry diagonal. Detailed analyses of each genetic lineages indicated strong population structure and no signs of isolation-by-distance. It thus seems that other environmental factors have shaped the genetic make-up of the Narrow-billed Woodcreeper populations.

The phylogenetic tree of the Narrow-billed Woodcreeper show that different genetic lineages correspond to particular biomes within the dry diagonal. Notice that the morphological groupings and subspecies are not supported by the genetic data. From: Rocha et al. (2020) Journal of Avian Biology.

Isolated Biomes

To determine which environmental factors influenced the evolution of the Narrow-billed Woodcreeper, the researchers reconstructed the past habitat of this species during the Pleistocene (between 2.5 million and 11,000 years ago). These analyses showed that the open biomes contracted during warmer interglacial periods and expanded during cold and
dry glacial periods. These dynamics suggest that certain populations became isolated during the interglacial periods and diverged genetically. When these biomes – and the populations within them – expanded again, the heterogeneity of the landscape prevented the populations from completely mixing again, giving rise to the genetic patterns we see today. The authors summarized their findings succinctly:

All genetic lineages identified here are historically associated with one stable climatic area suggesting that diversification within L. angustirostris has been primarily influenced by Pleistocene climatic oscillations, which promoted allopatric diversification during interglacial periods. Secondarily, the heterogeneous landscape along the dry diagonal may have being limiting gene flow among the genetic lineages after contact was re-established among them.

Present and past models of geographic distributions of the Narrow-billed Woodcreeper show that the different biomes were isolated in the past. Over time, the populations within these biomes diverged genetically. From: Rocha et al. (2020) Journal of Avian Biology.

Some Taxonomy

Finally, this study also has some taxonomic implications. The phylogenetic analyses indicated that none of the genetic lineages correspond to the recognized subspecies (which are based on morphological data). This result is in line with a recent review on the plumage variation within the Narrow-billed Woodcreeper, which can thus be seen as a single species with strong genetic structure.

References

Rocha, A. V., Cabanne, G. S., Aleixo, A., Silveira, L. F., Tubaro, P., & Caparroz, R. (2020). Pleistocene climatic oscillations associated with landscape heterogeneity of the South American dry diagonal explains the phylogeographic structure of the narrow‐billed woodcreeper (Lepidocolaptes angustirostris, Dendrocolaptidae). Journal of Avian Biology, 51(9).

Featured image: Narrow-billed Woodcreeper (Lepidocolaptes angustirostris) © Evaldo Resende | Wikimedia Commons