Genomic analysis of many nonmodel species has uncovered an incredible diversity of sex chromosome systems, making it possible to empirically test the rich body of evolutionary theory that describes each stage of sex chromosome evolution. Classic theory predicts that sex chromosomes originate from a pair of homologous autosomes and recombination between them is suppressed via inversions to resolve sexual conflict. The resulting degradation of the Y chromosome gene content creates the need for dosage compensation in the heterogametic sex. Sex chromosome theory also implies a linear process, starting from sex chromosome origin and progressing to heteromorphism. Despite many convergent genomic patterns exhibited by independently evolved sex chromosome systems, and many case studies supporting these theoretical predictions, emerging data provide numerous interesting exceptions to these long-standing theories, and suggest that the remarkable diversity of sex chromosomes is matched by a similar diversity in their evolution. For example, it is clear that sex chromosome pairs are not always derived from homologous autosomes. In addition, both the cause and the mechanism of recombination suppression between sex chromosome pairs remain unclear, and it may be that the spread of recombination suppression is a more gradual process than previously thought. It is also clear that dosage compensation can be achieved in many ways, and displays a range of efficacy in different systems. Finally, the remarkable turnover of sex chromosomes in many systems, as well as variation in the rate of sex chromosome divergence, suggest that assumptions about the inevitable linearity of sex chromosome evolution are not always empirically supported, and the drivers of the birth–death cycle of sex chromosome evolution remain to be elucidated. Here, we concentrate on how the diversity in sex chromosomes across taxa highlights an equal diversity in each stage of sex chromosome evolution.

There has been substantial interest of late in using population genetic methods to study sexual conflict, where an allele increases the fitness of one sex at some cost to the other (Mank, 2017). Population genomic scans for sexual conflict offer an important advance given the difficulties of identifying antagonistic alleles from more traditional methods, and could greatly increase our understanding of the extent and loci of sexual conflict. This is particularly true for studies in natural populations, for which obtaining accurate fitness measurements for each sex can be challenging. In this issue of Molecular Ecology, Bissegger, Laurentino, Roesti, and Berner (2019) present a cautionary tale about how to interpret these population genomic data.

Recent findings indicate that some teleost fishes may be able to greatly enhance hemoglobin-oxygen (Hb-O) unloading at the tissues under conditions that result in catecholamine release. The putative mechanism relies on the high pH sensitivity of teleost hemoglobin (Hb), intracellular red blood cell (RBC) pH regulation via β-adrenergic Na/H exchanger (β-NHE) activity, and plasma-accessible carbonic anhydrase at the tissues that short-circuits RBC pH regulation. Previous studies have shown that in rainbow trout, this system may double Hb-O unloading to red muscle compared to a situation without short-circuiting. The present study determined that: (1) in rainbow trout this system may be functional even at low concentrations of circulating catecholamines, as shown by conducting a dose-response analysis; (2) Atlantic and coho salmon also possess β-NHE activity, as shown by changes in hematocrit in adrenergically stimulated cells; and (3) with β-NHE short-circuiting, Atlantic and coho salmon may be able to increase Hb-O unloading by up to 74 and 159%, respectively, as determined by modeling based on O equilibrium curves. Together, these results indicate that a system to enhance Hb-O unloading may be common among salmonids and may be operational even under routine conditions. In view of the life histories of Atlantic and coho salmon, a system to enhance Hb-O unloading during exercise may help determine a successful spawning migration and thus reproductive success.

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White sturgeon (Acipenser transmontanus) in the Lower Fraser River are the focus of a catch-and-release angling fishery in British Columbia, Canada. However, the lower region of the catch area includes areas where tidal waters invade, and the consequence of salinity levels on recovery from an angling challenge are not characterized in sturgeon, despite theoretical implications of its import. We acclimated white sturgeon to various salinities (0, 10 and 20‰ (parts per thousand)) to investigate the effects of acclimation on recovery from stimulated angling stress that was induced through manual chasing. This challenge elicited the traditional physiological responses such as ion homeostasis disturbance, increases in secondary stress indicators and metabolic acidosis; however, environmental salinity altered the timing of recovery in some of the parameters measured. In addition, the severity of the intracellular pH disturbance in both heart and red blood cell seemed to be mediated in fresh water, yet the recovery pattern of plasma chloride and bicarbonate ions seemed to be facilitated by higher salinity. In general, responses were similar but not identical, leading us to conclude that the role of salinity on recovery from exercise is complex but not insignificant. Salinity may be important to behaviours exhibited by white sturgeon (such as migrations) in their respective saline environments, but less so around the impact of an angling stressor. Further exploration of this response may provide insight on whether the current tidal boundaries for angling white sturgeon are appropriate.