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.
Highlights
Trachymyrmex Forel is a New World genus of fungus-growing ants closely related to the genera Atta and Acromyrmex, the well-known leaf-cutting ants (Schultz and Brady 2008)
We identified two T. pakawa haplotypes for each marker (COI and the EF1-α-F1) (Figure 4)
The T. smithi specimens from Ojinaga, Chihuahua, Mexico, yielded COI and EF1-α-F1 sequences 100% and 99.99% identical to T. smithi Genbank accessions EF539730 and EF539779.1, respectively, from New Mexico, USA; these (New Mexico) and additional T. smithi Genbank sequences for both markers were used in our molecular analysis (Table 1)
Summary
Trachymyrmex Forel is a New World genus of fungus-growing ants closely related to the genera Atta and Acromyrmex, the well-known leaf-cutting ants (Schultz and Brady 2008). The genus Trachymyrmex is considered to have originated in tropical South America (Weber 1972, Mueller and Rabeling 2008); there has been at least one large species radiation of the genus in temperate North America, probably related to the Great American Interchange (Bagley and Johnson 2014) or even before the closure of the isthmus of Panama Based on morphological characters and a multilocus phylogenetic analysis, we describe a new species of the genus Trachymyrmex from limestonederived montane landscapes in the semiarid Nearctic of northeastern Mexico
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