| Sumario: | Abstract
Here we describe a new species of Trachymyrmex, T. pakawa sp. n., from the Gran Sierra Plegada range of
the Sierra Madre Oriental, in the states of Coahuila and Nuevo Leon, northeastern Mexico. Trachymyrmex
pakawa is a large-sized species compared to other North American Trachymyrmex. Its geographic distribution includes the piedmont of the Gran Sierra Plegada at La Estanzuela, Monterrey, as well as peripheral
mountains segregated from the Sierra Madre Oriental (Cerro de las Mitras, Sierra de Zapalinamé, Cañon
de San Lorenzo, Cerro de las Letras). The preferred habitats of T. pakawa include oak-pine forest at La
Estanzuela, xeric oak forest at Zapalinamé and mesic Chihuahuan desert scrub with sotol (Dasylirion)
at other sites. All localities are on slopes, on very rocky, shallow lithosols overlaying large boulders. This
species nests under and between large boulders and rocks. It has not been observed on alluvial or better
developed, deeper soils, and it is absent from sites with human activity (urban, disturbed, and landscaped
areas). It is closely related to and morphologically similar to Trachymyrmex smithi. The known distribution ranges of T. pakawa and T. smithi almost overlap in Saltillo, Coahuila state. The main character that
distinguishes the new species from T. smithi is longer antennal scapes in T. pakawa; also, different nesting habits (rocky slopes vs. alluvial sites or deep sand in T. smithi), and geographic distribution. Phylogenetic
analysis of DNA sequences from the mitochondrial marker cytochrome c oxidase subunit I (COI) and the
first intron of the F1 copy of the nuclear protein-coding gene Elongation Factor 1- α (EF1-α-F1) confirm
a sister-species relationship between T. pakawa and T. smithi. Bayesian coalescent analyses indicate a
divergence time of about 8.00 million years before present (95% confidence interval: 4.8–11.5 mya) between T. pakawa and T. smithi. The divergence of the lineages of T. pakawa and T. smithi could have been
driven by the Pliocene-Holocene desertification of southwestern North America. This process resulted
in isolated mesic refugia and forests in the Madrean ranges and piedmonts of northeastern Mexico (the
current habitat of T. pakawa) while T. smithi adapted to the deeper, often sandy soils on the drier desert
plains of Coahuila and Chihuahua states in Mexico, and New Mexico and Texas in the USA. Within the
Nearctic species of the Trachymyrmex septentrionalis species group, T. pakawa is the species that is closest
(by geographical distribution) to Neotropical species of Trachymyrmex like T. saussurei.
Keywords
Insecta, Attina, Myrmicinae, symbiosis
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