INTRODUCTION
Dispersal is a ubiquitous and fundamental life history trait that plays a role in the evolutionary ecology of organisms. Dispersal can shape population genetic differentiation, overall species distributions, and how species may respond or adapt to environmental change (Clobert, Baguette, Benton, & Bullock, 2012; Ellis et al., 2015). For species with low dispersal capabilities, historical, ecological, or anthropogenic barriers to movement can influence their population genetic structure over certain scales (Doña et al., 2019; Haye et al., 2014; Manel, Schwartz, Luikart, & Taberlet, 2003; Storfer, Murphy, Spear, Holderegger, & Waits, 2010). For example, barriers may slow gene flow between some populations, increase reproductive and genetic isolation, and ultimately result in genetically divergent populations (Clark, Brown, Stechert, & Zamudio, 2010; DiBlasi et al., 2018; Slatkin, 1987).
Within species, dispersal may be biased towards certain individuals, which has consequences for a species’ genetic diversity and population genetic structure. For example, in some species, female birds tend to disperse farther than males, while male mammals tend to disperse farther than females (sex-biased dispersal) (Greenwood, 1980; Trochet et al., 2016). While females form the core of societies in ants (and all social Hymenoptera), considerable variation exists among the dispersal abilities of males and reproductive females (Bourke & Franks, 1995; Cronin, Molet, Doums, Monnin, & Peeters, 2013; Hakala, Seppä, & Helanterä, 2019; Helms, 2018; Jacobs & Heinze, 2019; Keller, Peeters, & Beldade, 2014). For example, some ant species have wingless individuals (females or males) that can move across scales of just a few meters, or winged individuals that vary in their ability or tendency to disperse long distances (over hundreds of meters). Typically, dispersal pattern and colony founding mode determine intraspecific population genetic patterns. For example, in species where queens do not fly, males may fly and be responsible for most long-distance dispersal (Berghoff, Kronauer, Edwards, & Franks, 2008; Hakala et al., 2019). In other species, wingless males and winged females may experience inbreeding due to a lack of dispersal (Jacobs & Heinze, 2019). In species with winged males and females, both sexes may contribute to long-distance dispersal, gene flow, and near panmictic population structure (Johansson, Seppä, Helanterä, Trontti, & Sundström, 2018). On the other hand, some species with volant sexuals nevertheless show signatures of male-biased long-distance dispersal (Holzer, Keller, & Chapuisat, 2009), especially in species with relatively large and fat-laden queens that produce their first offspring via metabolic stores (claustral founding) (Helms, 2018).
The fungus-gardening (attine) ants are common ants in low to mid latitudes of the western hemisphere (Branstetter et al., 2017; Nygaard et al., 2016; Seal & Tschinkel, 2006). The ants depend on their fungal cultivar as their main nutritional source, and the fungus depends on the ants for propagation, survival, and dispersal. Virgin queens (gynes) are solely responsible for dispersal of the mutualistic fungi (vertical transmission) by storing it in an infrabuccal pocket prior to the nuptial flight (Huber, 1905; Mueller, 2002; Mueller, Schultz, Currie, Adams, & Malloch, 2001). Thus, symbiotic fungal expansion and diversification is expected to be largely dependent on dispersal capabilities of the female ants. As a result, female-biased dispersal (or at least not male-biased dispersal) may be favored in attine ants; strict male-biased dispersal would conflict with the dispersal interests of the fungus (Mueller, 2002) by limiting the expansion and diversification of the fungal symbiont. However, any conflicts could be resolved by males and gynes having similar dispersal abilities. Additionally, gynes of most attines appear to be energetically cheap to produce and most do not found claustrally (Fernández-Marín, Zimmerman, & Wcislo, 2004; Seal, 2009; Seal & Tschinkel, 2007b); thus, attines might not be under selection to produce large, heavy gynes that are generally dispersal-limited because of the energetic demands of flight (Helms, 2018). Consequently, the relatively lighter attine gynes may fly just as far as males, thus increasing expansion capabilities of the co-dispersed fungus.
Surprisingly, there have been very few studies that examine sex-biased dispersal and gene flow in fungus-gardening ants, even though the molecular markers necessary to do so have been developed. For instance, microsatellite markers have been used to examine polyandry, polygyny, and population structure (Bekkevold, Frydenberg, & Boomsma, 1999; Fjerdingstad & Boomsma, 2000; Helmkampf, Gadau, & Feldhaar, 2008; Kellner et al., 2013; Matthews, Rowan, Stone, Kellner, & Seal, 2020; Murakami, Higashi, & Windsor, 2000; Rabeling et al., 2013; Rabeling et al., 2011; Rabeling et al., 2014) and mitochondrial DNA (mtDNA) markers have been used to examine phylogeographic structure (i.e., female dispersal patterns) in several attine species (Cardoso, Cristiano, Tavares, Schubart, & Heinze, 2015; Seal, Thiebaud, & Mueller, 2015; Solomon, Bacci, Martins, Vinha, & Mueller, 2008). While at least one study reported comparable dispersal abilities in both male and female Atta colombica , only a small portion of its known range was surveyed (Helmkampf et al., 2008). Indeed,Atta species are the only attines that exhibit claustral founding (Fernández-Marín & Wcislo, 2005; Fernández-Marín et al., 2004; Hölldobler & Wilson, 2011; Huber, 1905); it therefore remains possible that the large females (among the largest individual ants in the world) are not capable of flying as far as males.
In this study, we employ mtDNA markers and diploid microsatellite markers that were developed using whole genome sequencing (Matthews et al., 2020) to examine population structure, gene flow, and signatures of sex-biased dispersal in a fungus-gardening ant species, Trachymyrmex septentrionalis(McCook, 1881) . T. septentrionalis is distributed from Texas to Florida to Long Island, New York, with a center of genetic diversity found in northern Florida (Rabeling, Cover, Johnson, & Mueller, 2007; Seal et al., 2015; Senula et al., 2019). Prior studies using mtDNA have shown significant phylogeographic structure across its range. For example, haplotypes are represented by phylogroups east and west of the Mississippi River, with western populations consisting of one clade whereas those found in the east are more diverse, consisting of at least three clades (Mikheyev, Vo, & Mueller, 2008; Seal et al., 2015). A preliminary analysis of nuclear microsatellites supports this overall pattern – higher diversity in populations east of the Mississippi River relative to the west (Matthews et al., 2020).T. septentrionalis is one of the most common and abundant ant species in the longleaf pine forests of Florida whose populations appear to respond rapidly to annual variations in climate (especially rainfall) and local conditions (e.g., relief) and often move significant amounts of soil in the process (Seal & Tschinkel, 2006, 2008, 2010; Tschinkel & Seal, 2016). Therefore, understanding the population genetics of this species will help us understand the dispersal biology of an ecologically important symbiosis.