The Subantarctic Rayadito (Aphrastura subantarctica), a new bird species on the southernmost islands of the Americas Page: 11
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have been favored on Diego Ramirez because it increases thermoregulation efficiency64. While a larger body size
has also been described in the subspecies (Aphrastura s. bullocki)13 and species (Aphrastura masafuerae)2,65,66
of Aphrastura inhabiting islands, the behavior related to grass habitat use and nesting at ground level seems to
be unique to Aphrastura on Diego Ramirez. Another subspecies inhabiting Patagonian islands is Aphrastura s.
fulva, but because the main differences from continental Aphrastura spinicauda are related to color patterns13, it
is also expected to be morphologically and behaviorally different from Aphrastura on Diego Ramirez. The find-
ings of our study are thus consistent with the suggestion made by Frangois Vuilleumier thirty years ago67, who
characterized Aphrastura spinicauda as a geographically variable species. He also pointed out that the individuals
observed on Diego Ramirez were morphologically and behaviorally different from northern populations, and
more similar to wrens (Troglodytidae) than to species ecologically equivalent to A. spinicauda, such as titmice
(Paridae) or treecreepers (Certhiidae).
The population on Diego Ramirez uses a distinct habitat that lacks woody plants, in ways that are unusual
when compared to the continental populations. The lack of trees with nesting cavities has led to an association
with breeding seabirds, which nest on Diego Ramirez during the austral spring and summer. Aphrastura indi-
viduals use burrows of different seabird species, sometimes associating with larger seabirds (e.g., Grey-headed
albatross), which may provide protection from weather and potential aerial predators (e.g., Striated caracara Phal-
coboenus australis and the Chilean skua Stercorarius chilensis). Across the entire range of Aphrastura spinicauda,
individuals respond to intruders near their nest sites with alarm calls. In the continental populations, mobbing
calls have minimum and maximum frequencies of 2.82 and 13.01 kHz, respectively, with at least six notes per
second in central and southern Chile68. However, based on a previous record in 2001, it seems that minimum and
maximum frequencies of mobbing calls of Aphrastura subantarctica are lower, with the same number of notes
(see69). While these preliminary observations have to be confirmed by future studies, the low call frequencies
of Aphrastura on Diego Ramirez could be related to the high ambient noise, as well as their larger body size70.
Genetic differentiation. The genetic structure resulting from both mitochondrial and autosomal data sug-
gests that the Aphrastura population from Diego Ramirez represents an isolated entity. The autosomal differenti-
ation is substantial even when considering values of the G'ST(Nei) index, which is a more conservative statistic than
Hedrick's index71. Despite sharing a mitochondrial haplotype with populations from Navarino and the Horn
islands, the Aphrastura population of Diego Ramirez is also differentiated at the mitochondriallevel, as shown by
the significant genetic distance between them and individuals in Navarino and the Horn islands. Furthermore,
Diego Ramirez appeared as a genetically separated cluster in our analyses, indicating a complete absence of sub-
sequent, or at least recent, migration to or from the archipelago. When excluding the Diego Ramirez individuals,
the other populations of A. spinicauda have no clear genetic structure at the autosomal and mitochondrial level
across their distribution, which is consistent with moderate to high levels of gene flow, as documented by other
authors30. Our results support the recent observation of the absence of current gene flow between the Diego
Ramirez population and their relatives inhabiting the continent and islands closer to it10. This population could
thus be considered a separate evolutionary unit.
The sub-Antarctic Magellanic ecoregion experienced variations in sea level and volume of ice sheet during the
Last Glacial Maximum (LGM, 18,000-28,000 years BP)72,73, which has resulted in a heterogeneous environment
with high level of endemism across various biological groups74. The population on Diego Ramirez was probably
established from a unique colonization event of a small group of individuals that originated from the southern
Patagonian islands, which are geographically nearest to Diego Ramirez, such as Navarino or Horn islands, with
one shared mitochondrial haplotype among them suggesting this historical event. In addition, the presence of
a single cytb haplotype on Diego Ramirez suggests a strong founder effect, while the absence of new private
haplotypes suggests a scenario of recent, post-LGM colonization of Diego Ramirez. The noticeably low levels
of heterozygosity, allelic richness, and number of private alleles in the microsatellite data further supports this
hypothesis. In evolutionary terms, the relatively short time that has passed since a post-LGM colonization event
might have been sufficient for the Diego Ramirez population to genetically differentiate from A. spinicauda. The
absence of the main mitochondrial haplotype present in all A. spinicauda populations, from the northernmost
to the southernmost ones, suggests that the population on Diego Ramirez is undergoing a peripatric divergence
from A. spinicauda.
A new taxonomic unit. Our analyses indicate that the Aphrastura population from Diego Ramirez com-
pose a demographically independent population that should be considered as a new conservation and taxonomic
unit. This population has remained isolated and encompasses specific morphological, genetic, and behavioral
features, for which reason it is now proposed as a new species75. In his influential paper, de Queiroz76 reconciled
the concept of species and stated that the only nonnegotiable condition for a species to be considered as such is
to separately evolve from another lineage, which according to our data has taken place for the Aphrastura popu-
lation in Diego Ramirez. Whether there is complete lineage sorting and intrinsic reproductive isolation among
the Aphrastura populations or not will only show how far the diverging process has gone through the speciation
continuum (e.g.,77).
We have presented multiple pieces of evidence suggesting the existence of a new species by combining
morphological, ecological and genetic data, and we argue that this integrative framework is crucial for defining
new taxonomic units. Acoustic analysis, a powerful tool for supporting species limits in suboscine passerines,
including Furnariidae51, may provide more evidence of differentiation of the Diego Ramirez population. Further
research may include samples from A. masafuerae and A. s. bullocki, as well as full genomes for evaluating the
genomic landscape of divergence during the speciation process of the genus Aphrastura. Several other bird taxa
with small genetic differences have been reclassified and subsequently considered as valid species, such as thehttps://doi.org/10.1038/s41598-022-17985-4
Scientific Reports (2022) -12:13957
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Rozzi, Ricardo, 1960-; Quilodrán, Claudio S.; Botero-Delgadillo, Esteban; Napolitano, Constanza; Torres-Mura, Juan C.; Barroso, Omar et al. The Subantarctic Rayadito (Aphrastura subantarctica), a new bird species on the southernmost islands of the Americas, article, August 26, 2022; (https://digital.library.unt.edu/ark:/67531/metadc2178792/m1/11/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Science.