How phenotypic diversity originates and persists within populations are classic puzzles in evolutionary biology. While balanced polymorphisms segregate within many species, it remains rare for both the genetic basis and the selective forces to be known, leading to an incomplete understanding of many classes of traits under balancing selection. Here, we uncover the genetic architecture of a balanced sexual mimicry polymorphism and identify behavioral mechanisms that may be involved in its maintenance in the swordtail fish Xiphophorus birchmanni. We find that ∼40% of X. birchmanni males develop a "false gravid spot," a melanic pigmentation pattern that mimics the "pregnancy spot" associated with sexual maturity in female live-bearing fish. Using genome-wide association mapping, we detect a single intergenic region associated with variation in the false gravid spot phenotype, which is upstream of kitlga, a melanophore patterning gene. By performing long-read sequencing within and across populations, we identify complex structural rearrangements between alternate alleles at this locus. The false gravid spot haplotype drives increased allele-specific expression of kitlga, which provides a mechanistic explanation for the increased melanophore abundance that causes the spot. By studying social interactions in the laboratory and in nature, we find that males with the false gravid spot experience less aggression; however, they also receive increased attention from other males and are disdained by females. These behavioral interactions may contribute to the maintenance of this phenotypic polymorphism in natural populations. We speculate that structural variants affecting gene regulation may be an underappreciated driver of balanced polymorphisms across diverse species.

Objectives

To assess the reliability and the minimal clinically important differences (MCID) for FVC% predicted in the Scleroderma Lung Study I and II.

Methods

Using data from SLS I and II (N=300), we evaluated the test-retest reliability for FVC% predicted (FVC%; screening vs. baseline) using intra-class correlation (ICC). MCID estimates at 12 months were calculated in the pooled cohort (SLS-I and II) using 2 anchors: Transition Dyspnea Index (≥change of 1.5 units for improvement and worsening, respectively) and the SF-36 Health Transition question: "Compared to one year ago, how would you rate your health in general now?", where "somewhat better" or "somewhat worse" were defined as the MCID estimates. We next assessed the association of MCID estimates for improvement and worsening of FVC% with patient reported outcomes (PROs) and computer-assisted quantitation of extent of fibrosis (QLF) and of total ILD (QILD) on HRCT.

Results

Reliability of FVC%, assessed at a mean of 34 days, was 0.93 for the pooled cohort. The MCID estimates for the pooled cohort at 12 months for FVC% improvement ranged from 3.0 % to 5.3% and for worsening from -3.0% to -3.3%. FVC% improvement by ≥MCID was associated with either statistically significant or numerical improvements in some PROs, QILD, and QLF, while FVC% worsening ≥MCID was associated with statistically significant or numerical worsening of PROs, QILD, and QLF.

Conclusion

FVC% has acceptable test-retest reliability, and we have provided the MCID estimates for FVC% in SSc-ILD based changes at 12 months from baseline in two clinical trials. Clinical trial registration available at www.clinicaltrials.gov, IDs NCT00004563 and NCT00883129.