Book Description

Threespine sticklebacks (Gasterosteus aculeatus) are named for their characteristic dorsal spines, but different populations show variation in spine size and number. Marine populations typically have long spines that form part of their defensive armor, whereas many freshwater populations exhibit reduction or loss of the spines. In a large F2 cross used previously for QTL mapping, dorsal spine length maps to chromosome 4, to a locus that contains MSX2A. An enhancer sequence near MSX2A drives expression in the spines, and the MSX2A transcript shows allele-specific expression bias in F1 hybrids. However, the expression bias appears to be primarily caused by a difference in mRNA splicing. A single-nucleotide change in freshwater fish creates a predicted hnRNP binding site that favors the formation of a much smaller transcript. In F1 hybrids, the full-length transcripts are produced mainly from the marine allele, and short transcripts are produced from the freshwater allele. Reintroduction of the marine version of the gene significantly restores spine length in transgenic freshwater fish. Thus, the reduced spines of freshwater sticklebacks appear to have evolved in part due to the appearance of a new functional element affecting splicing. Additionally, the lack of MSX2A in certain fish species that have lost their bony armor suggests that the gene may have played a role in other instances of armor reduction in teleosts. Additional QTL for spine length and other skeletal traits were identified in a new cross between sticklebacks from Boulton Lake, British Columbia, and Bodega Bay, California. The Boulton Lake population differs from other freshwater populations in the nature of its dorsal spine and pelvic reduction. The MSX2A locus was again the major QTL identified for dorsal spine length, but the set of minor QTL included a new locus at the end of chromosome 6 that affected spine 2 specifically. Pelvic spine length was linked to the MSX2A region of chromosome 4 in this cross, not to the major locus on chromosome 7 observed in several other populations, and there was evidence for additional QTL on chromosome 4 and on 9 other chromosomes. Among the F2s of the new cross were fish carrying an extra dorsal spine. The presence of this 4th spine was mapped as a binary trait, and QTL were found on chromosomes 1 and 6. Although the Boulton Lake and Bodega Bay cross parents had 2 spines and 3 spines, respectively, the 4-spine phenotype was associated with Boulton genotypes at both QTL. These results suggest that, in the context of marine genotypes at certain other loci, Boulton freshwater alleles can increase spine number beyond the range observed in either parental population.