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Phylogenetic relationships, geographic distribution, and morphological stasis of gars. (A) Tip-dated phylogenetic tree of gars based on three subsets of the 90 largest exons and with positions of fossils fixed based on morphological phylogenies. The red circle marks the timing of divergence between Atractosteus and Lepisosteus at approximately 105 million years ago. Phylogeny from Brownstein et al. (2023). (B) Morphological stasis in gars is exemplified by nearly identical species pairs separated by over 50 million years. Photographs of gar fossils are by Lance Grande, and photographs of living gars are by Zachary Miller, both used with permission.
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Evolutionary stasis characterizes lineages that seldom speciate and show little phenotypic change over long stretches of geological time. Although lineages that appear to exhibit evolutionary stasis are often called living fossils, no single mechanism is thought responsible for their slow rates of morphological evolution and low species diversity....
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Evolutionary stasis characterizes lineages that seldom speciate and show little phenotypic change over long stretches of geological time. Although lineages that appear to exhibit evolutionary stasis are often called living fossils, no single mechanism is thought to be responsible for their slow rates of morphological evolution and low species diver...
Citations
... The species in the linkage map collection belong to the main extant fish lineage, teleosts [20,31], except the outgroup species, spotted gar, that has been termed a living fossil [11] (Figure 1). Teleost fishes have two small and two large subdivisions Euteleostei and Otocephala. ...
Meiotic recombination is an ubiquitous feature of sexual reproduction across eukaryotes. While recombination has been widely studied both theoretically and experimentally, the causes of its variation across species are still poorly understood. Composing a coherent view across species has been difficult because of the differences in recombination map generation and reporting of the results. Thus, fundamental questions like why recombination rates differ between sexes (heterochiasmy) in many but not all species remain unanswered. Here we present the first collection of recombination maps that allows quantitative comparisons across a diverse set of species. We generated sex-specific high-density linkage maps for 40 fish species using the same computational pipeline. Comparing the maps revealed that the higher genome-wide recombination rate in females compared to males was linked to the karyotype of the species. The difference between the sexes in the positioning of the crossovers was also highly variable and unrelated to the difference in their total number. Especially in males, CpG content of the sequence was a strong indicator of the broad scale distribution of crossovers between and within chromosomes. More generally, the collection of recombination landscapes can serve as a link between the theoretical and experimental work on recombination.
... Gene flow through assortative mating is, however, not completely halted in our model (the probability of gene flow ranges between 1 and 5% depending on the degree of specialization required to successfully feed on the food resources). Recurrent gene flow among ecologically diverse species is common in the wild 58 , and according to recent research, it can play a major role in promoting diversification 3 . ...
The presence or absence of sex can have a strong influence on the processes whereby species arise. Yet, the mechanistic underpinnings of this influence are poorly understood. To gain insights into the mechanisms whereby the reproductive mode may influence ecological diversification, we investigate how natural selection, genetic mixing, and the reproductive mode interact and how this interaction affects the evolutionary dynamics of diversifying lineages. To do so, we analyze models of ecological diversification for sexual and asexual lineages, in which diversification is driven by intraspecific resource competition. We find that the reproductive mode strongly influences the diversification rate and, thus, the ensuing diversity of a lineage. Our results reveal that ecologically-based selection is stronger in asexual lineages because asexual organisms have a higher reproductive potential than sexual ones. This promotes faster diversification in asexual lineages. However, a small amount of genetic mixing accelerates the trait expansion process in sexual lineages, overturning the effect of ecologically-based selection alone and enabling a faster niche occupancy than asexual lineages. As a consequence, sexual lineages can occupy more ecological niches, eventually resulting in higher diversity. This suggests that sexual reproduction may be widespread among species because it increases the rate of diversification.
... Having diverged from bony fish 200 million years ago, sturgeon have remained relatively unchanged morphologically (Bemis, Findeis, and Grande 1997) and genetically (Brownstein et al. 2024) since the lower Jurassic. Yet Acipenseriformes have very high levels of ploidy. ...
... As sturgeon are long-lived species with relatively few genomic changes occurring through millions of years of evolution (Brownstein et al. 2024), individuals likely rely on their ability to acclimate to environmental changes in real time, which necessitates their impressive plasticity in the face of increased environmental temperatures. During early development, sturgeon appear to have higher acclimation capacities compared to later life stages, which may aid during a period of limited mobility when effective behavioral thermoregulation (e.g., swimming far distances to avoid high temperatures) is not feasible Verhille et al. 2014;Brandt et al. 2021). ...
Understanding how ectotherms may fare with rising global temperatures and more frequent heatwaves is especially concerning for species already considered at‐risk, such as long‐lived, late‐maturing sturgeon. There have been concerted efforts to collect data on the movement behavior and thermal physiology of North American sturgeon to enhance conservation efforts; thus, we sought to synthesize these data to understand how sturgeon respond to thermal stress and what capacity they have to acclimate and adapt to warming. Here, we combined a systematic literature review and meta‐analysis, integrating field‐based observations (distribution and spawning) and laboratory‐based experiments (survival, activity, growth, metabolism, and upper thermal limits) for large‐scale insights to understand the vulnerability of North American sturgeon to rising global temperatures. We summarized the preferred thermal habitat and thermal limits of sturgeon in their natural environment and using meta‐analytical techniques, quantified the effect of prolonged temperature change on sturgeon whole‐animal physiology and acute upper thermal limits. While acclimation did not have significant effects on physiological rates or survival overall, there were positive trends of activity and metabolism in young‐of‐the‐year sturgeons, likely offset by negative trends of survival in early life. Notably, North American sturgeon have a greater capacity for thermal tolerance plasticity than other fishes, increasing upper thermal limits by 0.56°C per 1°C change in acclimation temperature. But with limited laboratory‐based studies, more research is needed to understand if this is a sturgeon trait, or perhaps that of basal fishes in general. Importantly, with these data gaps, the fate of sturgeon remains uncertain as climate change intensifies, and physiological impacts across life stages likely limit ecological success.
