Evolution of stickleback spines through independent cis-regulatory changes at HOXDB

Understanding the genetic mechanisms leading to new traits is a fundamental goal of evolutionary biology. We show that HOXDB regulatory changes have been used repeatedly in different stickleback fish species to alter the length and number of bony dorsal spines. In Gasterosteus aculeatus, a variant HOXDB allele is genetically linked to shortening an existing spine and adding a spine. In Apeltes quadracus, a variant allele is associated with lengthening an existing spine and adding a spine. The alleles alter the same conserved non-coding HOXDB enhancer by diverse molecular mechanisms, including SNPs, deletions, and transposable element insertions. The independent cis-acting regulatory changes are linked to anterior expansion or contraction of HOXDB expression. Our findings support the long-standing hypothesis that natural Hox gene variation underlies key morphological patterning changes in wild populations and illustrate how different mutational mechanisms affecting the same region may produce opposite gene expression changes with similar phenotypic outcomes.

Food web structure within an estuary of the southern Gulf of St. Lawrence undergoing eutrophication

This study compared food web structure in eutrophied Ulva lactuca-dominated areas within an estuary in the Southern Gulf of St. Lawrence, with Zostera marina beds within the same estuary. The estuarine food web consisted only of primary producers, mesograzers, and secondary grazers, with the absence of piscivorous top predators. It was hypothesized that the altered plant habitat would lead to structural changes in the food web and the dominance of benthic carbon. Stomach contents from mummichog (Fundulus heteroclitus), fourspine stickleback (Apeltes quadracus), and American eel (Anguilla rostrata) showed that only mummichog had significant differences in prey items between the different habitats. Stable isotopes showed that there were no significant differences in the food web structure and individual species’ 13C values. A 13C spike in particulate organic matter during the onset of anoxia in July, presumably due to bacterial blooms, indicated the complete dominance of benthic carbon the pelagic food web during this month. Thus, blooms of heterotrophs during anoxic events may have the greatest influence on nutrient cycling in estuaries undergoing eutrophication.

Anti-Predator Responses To Conspecific and Heterospecific Skin Extracts By Threespine Sticklebacks: Alarm Pheromones Revisited

Under laboratory conditions, we investigated the presence of chemical alarm signals in the threespine stickleback (Gasterosteus aculeatus). We exposed individual threespine sticklebacks to skin extract of conspecifics originating from either the same or a different population, fourspine sticklebacks (Apeltes quadracus; a member of the same prey guild as the threespine stickleback) or swordtails (Xiphophorus helleri), a species not known to possess alarm pheromones and which is phylogenetically distant and allopatric from the threespine stickleback. Threespine sticklebacks exhibited significant increases in anti-predator behaviour patterns when presented with skin extract from both populations of conspecifics and from fourspine sticklebacks, but not to swordtail skin extract. These results suggest, contrary to previous reports, that threespine sticklebacks possess chemical alarm signals, which appear to be similar to those of Ostariophysan fishes.

The adaptive significance of dorsal spine variation in the fourspine stickleback, Apeltes quadracus. V. Temporal variation

We investigated temporal variation in polymorphisms for the number of dorsal spines in the fourspine stickleback, Apeltes quadracus, and in the ninespine stickleback, Pungitius pungitius, to complement studies based on geographic variation. The changes in spine number that occurred over a 10-year period at Daigle Inlet, New Brunswick, are small relative to geographic variation among sites in the Maritime Provinces. However, some statistically significant changes occur in both species: they take place at or near reproduction; there is no evidence that they are related to selection by predators or any other environmental factor that we measured; and they tend to be followed by reversals that damp the net change. Contrary to expectation, based on spatial relationships, spine numbers in A. quadracus and P. pungitius do not covary predictably over time. The results show that events at or near reproduction play a role in determining local morph frequencies. Our main finding is that spine number is surprisingly stable for both species, and we conclude that it is constrained to local equilibrium values.