Inferred genetic architecture underlying evolution in a fossil stickleback lineage

Inferring the evolution of reproductive isolation in a lineage of fossil threespine stickleback, Gasterosteus doryssus

Darwin attributed the absence of species transitions in the fossil record to his hypothesis that speciation occurs within isolated habitat patches too geographically restricted to be captured by fossil sequences. Mayr's peripatric speciation model added that such speciation would be rapid, further explaining missing evidence of diversification. Indeed, Eldredge and Gould's original punctuated equilibrium model combined Darwin's conjecture, Mayr's model and 124 years of unsuccessfully sampling the fossil record for transitions. Observing such divergence, however, could illustrate the tempo and mode of evolution during early speciation. Here, we investigate peripatric divergence in a Miocene stickleback fish, Gasterosteus doryssus. This lineage appeared and, over approximately 8000 generations, evolved significant reduction of 12 of 16 traits related to armour, swimming and diet, relative to its ancestral population. This was greater morphological divergence than we observed between reproductively isolated, benthic-limnetic ecotypes of extant Gasterosteus aculeatus. Therefore, we infer that reproductive isolation was evolving. However, local extinction of G. doryssus lineages shows how young, isolated, speciating populations often disappear, supporting Darwin's explanation for missing evidence and revealing a mechanism behind morphological stasis. Extinction may also account for limited sustained divergence within the stickleback species complex and help reconcile speciation rate variation observed across time scales.

Miocene phytolith and diatom dataset from 10.3Myo diatomite formation, Fernley, Nevada, USA

Phytoliths are opal silica particles formed within plant tissues. Diatoms are aquatic, single-celled photosynthetic algae with silica skeletons. Phytolith and diatom morphotypes vary depending on local environmental and climatic conditions and because their silicate structures preserve well, the study of phytolith and diatom morphotypes can be used to better understand paleoclimatic and paleoenvironmental dynamics and changes. This article presents original data from an 820cm-deep stratigraphy excavated at the Hazen diatomite deposits, a high-elevation desert paleolake in the Fernley District, Northern Nevada, USA. The site has been studied for an assemblage of fossilized threespine stickleback, Gasterosteus doryssus, that reveal adaptive evolution. For this study, a total of 157 samples were extracted at 20 cm intervals covering approximately 24,500 years. After extraction, the samples were mounted on slides and viewed under 400-1000x light microscopy, enabling classification of 14 phytolith and 45 diatom morphotypes. Our data support paleoenvironmental reconstructions of the Hazen Miocene paleolake.

A Behavioral Syndrome Linking Boldness and Flexibility Facilitates Invasion Success in Sticklebacks

AbstractFor a species to expand its range, it needs to be good at dispersing and also capable of exploiting resources and adapting to different environments. Therefore, behavioral and cognitive traits could play key roles in facilitating invasion success. Marine threespined sticklebacks (Gasterosteus aculeatus) have repeatedly colonized freshwater environments and rapidly adapted to them. Here, by comparing the behavior of hundreds of lab-reared sticklebacks from six different populations, we show that marine sticklebacks are bold, while sticklebacks that have become established in freshwater lakes are flexible. Moreover, boldness and flexibility are negatively correlated with one another at the individual, family, and population levels. These results support the hypothesis that boldness is favored in invaders during the initial dispersal stage, while flexibility is favored in recent immigrants during the establishment stage, and they suggest that the link between boldness and flexibility facilitates success during both the dispersal stage and the establishment stage. This study adds to the growing body of work showing the importance of behavioral correlations in facilitating colonization success in sticklebacks and other organisms.

Attempting genetic inference from directional asymmetry during convergent hindlimb reduction in squamates

Loss and reduction in paired appendages are common in vertebrate evolution. How often does such convergent evolution depend on similar developmental and genetic pathways? For example, many populations of the threespine stickleback and ninespine stickleback (Gasterosteidae) have independently evolved pelvic reduction, usually based on independent mutations that caused reduced Pitx1 expression. Reduced Pitx1 expression has also been implicated in pelvic reduction in manatees. Thus, hindlimb reduction stemming from reduced Pitx1 expression has arisen independently in groups that diverged tens to hundreds of millions of years ago, suggesting a potential for repeated use of Pitx1 across vertebrates. Notably, hindlimb reduction based on the reduction in Pitx1 expression produces left-larger directional asymmetry in the vestiges. We used this phenotypic signature as a genetic proxy, testing for hindlimb directional asymmetry in six genera of squamate reptiles that independently evolved hindlimb reduction and for which genetic and developmental tools are not yet developed: Agamodon anguliceps, Bachia intermedia, Chalcides sepsoides, Indotyphlops braminus, Ophisaurus attenuatuas and O. ventralis, and Teius teyou. Significant asymmetry occurred in one taxon, Chalcides sepsoides, whose left-side pelvis and femur vestiges were 18% and 64% larger than right-side vestiges, respectively, suggesting modification in Pitx1 expression in that species. However, there was either right-larger asymmetry or no directional asymmetry in the other five taxa, suggesting multiple developmental genetic pathways to hindlimb reduction in squamates and the vertebrates more generally.

(Non)Parallel developmental mechanisms in vertebrate appendage reduction and loss

Appendages have been reduced or lost hundreds of times during vertebrate evolution. This phenotypic convergence may be underlain by shared or different molecular mechanisms in distantly related vertebrate clades. To investigate, we reviewed the developmental and evolutionary literature of appendage reduction and loss in more than a dozen vertebrate genera from fish to mammals. We found that appendage reduction and loss was nearly always driven by modified gene expression as opposed to changes in coding sequences. Moreover, expression of the same genes was repeatedly modified across vertebrate taxa. However, the specific mechanisms by which expression was modified were rarely shared. The multiple routes to appendage reduction and loss suggest that adaptive loss of function phenotypes might arise routinely through changes in expression of key developmental genes.

Threespine Stickleback: A Model System For Evolutionary Genomics

The repeated adaptation of oceanic threespine sticklebacks to fresh water has made it a premier organism to study parallel evolution. These small fish have multiple distinct ecotypes that display a wide range of diverse phenotypic traits. Ecotypes are easily crossed in the laboratory, and families are large and develop quickly enough for quantitative trait locus analyses, positioning the threespine stickleback as a versatile model organism to address a wide range of biological questions. Extensive genomic resources, including linkage maps, a high-quality reference genome, and developmental genetics tools have led to insights into the genomic basis of adaptation and the identification of genomic changes controlling traits in vertebrates. Recently, threespine sticklebacks have been used as a model system to identify the genomic basis of highly complex traits, such as behavior and host-microbiome and host-parasite interactions. We review the latest findings and new avenues of research that have led the threespine stickleback to be considered a supermodel of evolutionary genomics.