Electrically evoked compound action potentials in adult and pediatric cochlear implant users: II. Assessing cochlear nerve health status with the interphase gap effect and the phase locking value.

Scientists have long been captivated by the repeatability of evolution. Iconic examples of such parallelism include repeated loss of pigmentation and vision in cave fauna, myriad flight-loss events in birds and insects, and rapid ecotype shifts in sticklebacks. Some of these intriguing cases involve complex multi-trait shifts, underpinned by diverse loci. While recent data suggest genomic architecture can potentially expedite such shifts, evolutionary theory still struggles to explain how complex ecotypes can evolve so rapidly (e.g. over ecologically-relevant timeframes). Globally, transitions from freshwater-marine migratory (diadromous) to freshwater-limited (landlocked) life-histories have been key drivers of fish diversification. However, the mechanisms underpinning the repeated evolution of phenotypically similar landlocked lineages have remained contentious, as adaptation has produced concordant shifts across diverse traits such as spawning behaviour, osmoregulation, reproductive investment, and predator avoidance. Researchers initially proposed that such multi-trait shifts must be explained by selection at unlinked loci, with disparate freshwater-adapted alleles introgressed into migratory populations, and subsequently re-aggregated in newly landlocked populations via recombination (the ‘transporter hypothesis’; Fig. ). However, emerging data call this model into question, suggesting instead that rapid parallel adaptation may be facilitated by rare migratory ‘jackpot’ individuals carrying co-located landlocking alleles within large recessive haploblocks (Fig. ). Nonetheless, empirical studies have found scant (if any) evidence for such ‘landlocking’ haploblocks within migratory fish populations. Landlocking of marine-migratory fishes drives parallel shifts in morphology, behaviour, and reproduction (Fig. ). The ‘transporter’ hypothesis proposes that multiple important ‘landlocking’ alleles are recruited independently across large numbers of marine-migratory founders. In contrast, the ‘jackpot’ hypothesis involves the simultaneous recruitment of multiple landlocking alleles as haploblocks carried by key migratory individuals (purple). Unlike the ‘transporter’ hypothesis, the ‘jackpot’ hypothesis provides a plausible mechanism for rapid ecotype shifts in response to sudden environmental change, such as in numerous New Zealand lakes formed by catastrophic landslides. Under both scenarios, ‘landlocking’ alleles are re-distributed back to the marine-migratory population via introgressive hybridisation (dashed arrow), but these are more greatly fragmented by recombination under the transporter than under the jackpot hypothesis. Our study of New Zealand provides evidence for high frequencies of ‘jackpot’ individuals in marine-migratory populations, enabling rapid ecotype shifts even when lake founding sizes are small (Fig. ) (images: DigitalGlobe; ).