Sampling
To understand the genetic architecture of mitonuclear incompatibilities we focused on two well-studied populations of Tigriopus californicus : San Diego (SD) and Santa Cruz (SC). These populations were suitable for a competition experiment between divergent nuclear alleles under a fixed mitochondrial background because previous studies have shown that: i) marked mitochondrial divergence (22.17%; Pereira et al. 2016) has driven adaptive evolution targeting genes functionally interacting with the mitochondria (Barreto & Burton 2013a), ii) 91% of the nuclear polymorphisms are fixed among parental populations (Pereira et al. 2016) allowing an accurate estimation of allelic frequencies in hybrids (Lima et al. 2019), iii) F2 hybrids show breakdown at all physiological and life history traits known to be associated with mitonuclear incompatibilities (Ellison & Burton 2006; a; b; Barreto & Burton 2013b; Barreto et al. 2015), and iv) strong selection during F2 development results in significant deviations from the expected Mendelian inheritance (Foley et al.2013; Healy & Burton 2020).
Stock populations were collected in San Diego (32◦ 44′ 41.17′′ N, 117◦ 15′ 19.43′′ W) and Santa Cruz (36◦ 56′ 58.32′′ N, 122◦ 2′ 48.98′′ W). Multiple cultures from each site were maintained in 400 mL beakers at common garden at 20°C with a 12:12 L:D photoperiod, in filtered seawater mixed with ground “Algae Wafers” (Kyorin Co., Himeji, Japan). Beakers were monthly mixed to promote panmixia within populations and medium was renewed.