Avoidance of inbred or unhealthy partner
Although an equal number of pairings for every pair of colonies was experimentally set up, detection and avoidance of partners who are either susceptible (those with high microbial loads) or nestmates potentially occurs during nuptial flights, discouraging random pairing in the field and minimizing the chance of pairing with a weak partner. We originally planned to test whether the choice of alates in this study relies on their level of relatedness, microbial similarity and load (similar to (Li et al. 2013; Sinotte et al. 2021)). However, partner choice was inconsistent as alates either engaged in tri-tandem running or continuously changing partners (pers. obs .). To date, evidence of detection and avoidance of nestmate pairings are scarce and inconsistent in termites. Inbreeding avoidance can occur through a split sex-ratio between colonies, or differences between the sexes in their dispersal range or in their timing of emergence. However, these indirect mechanisms are rare among termites ((Aguilera-Olivares et al. 2015) but see (Husseneder et al. 2006; Miyaguni et al. 2021)). Long-range dispersal is probably the predominant mechanism preventing inbreeding in many species, as alates can disperse hundreds of meters (Mullins et al. 2015; Zhang et al. 2021), which leads to genetic differentiation between closely located populations/colonies (Shellman-Reeve 2001; Fougeyrollas et al. 2018). Alates of most species do not seem to discriminate against nestmates, although this mechanism has been poorly studied (Vargo & Husseneder 2011). Non-random matings despite long-range dispersal has been occasionally reported, with inbreeding avoidance in R. chinensis (Li et al.2013), but preference in Coptotermes lacteus (Thompson et al. 2007) and R. flavipes (DeHeer & Vargo 2006). Together with the large variation of relatedness between partners uncovered within and among species, and at different stages of the colony lifecycle (Vargo & Husseneder 2011; Vargo 2019), our findings also support the conclusion that inbreeding avoidance is probably not a prime determinant of partner choice in termites during colony foundation (Sinotte et al.2021).
Similarly, there is little evidence of detection and avoidance of unhealthy alates in termites, despite the fact that pathogen avoidance is commonly documented in workers (Hussain et al. 2010; Tranteret al. 2014; Yanagawa et al. 2015). In R. chinensis, alates paired less frequently with an injured partner (Liet al. 2013), but females of Z. angusticollis showed no preference for healthy males rather than males infected withMetarhizium (Rosengaus et al. 2011a). Our results revealed that the high risk of pairing with a sick partner represents most of the mortality observed during colony foundation, which suggests that pathogen recognition and avoidance should act as a strong selective force. This selection should not only be based on the detection of the external presence of spores, but on an overall evaluation of partner health, such as changes in behavior or cuticular hydrocarbons (Beaniet al. 2019) (Supplementary Information S2). However, the influence of other potential selective pressures associated with nuptial flights (e.g., non-mating, predation and resource shortage) may instead lead partners to choose the first mate they encounter, regardless of their relatedness or health (Bengtsson 1978; Waseret al. 1986; Lehmann & Perrin 2003).