Why Did Kingsley and His Team Cross Marine and Freshwater Sticklebacks

Why Did Kingsley and His Team Cross Marine and Freshwater Sticklebacks

Earlier this year, Kingsley and his team of researchers crossed freshwater and marine sticklebacks in an effort to improve the survival of this species.

Their goal was to improve the chances of survival of the freshwater species and to increase the reproductive success of the marine species.

Evolution of sticklebacks

Several species of marine and freshwater sticklebacks have evolved from ancestral fish types.

These fish exhibit characteristic behavior and adaptations to living in marine and freshwater environments.

These adaptations are often studied by comparing present-day marine and freshwater populations. However, it is not always easy to identify the multiple linked loci controlling different traits.

Researchers disentangled patterns of parallel evolution of threespine sticklebacks living in freshwater environments.

They found extraordinary levels of genetic parallelism in the Eastern Pacific region.

These results suggest that marine and freshwater sticklebacks may have diverged in evolutionary time, but that the traits they exhibit have not changed drastically.

Researchers examined the genomes of 21 species of sticklebacks from three continents.

They found that most of the adaptations occurred within regulatory DNA, but that 20% were situated within coding DNA.

These adaptations are attributed to the selection pressures imposed by local predation. These forces have caused stickleback populations to repeatedly evolve.

Sticklebacks are widely distributed throughout the northern hemisphere.

They are mainly found in freshwater environments, though they can also be found in oceanic habitats.

They have a wide range of adaptations, including reduced armor. Some freshwater populations have lost dorsal and pelvic spines.

Others have reduced their skeletal armor, which may be linked to decreased calcium and vertebrate predation.

The evolution of marine and freshwater sticklebacks has occurred over many millions of years, resulting in a large number of distinct populations.

This diversity is reflected in the presence of different spine numbers and coloration patterns.

The number of spines can vary dramatically between species.

During breeding, male sticklebacks produce tubular nests of organic material glued together with kidney secretions.

These nests are used by the males to form territories. They move into shallow littoral areas in early spring.

Hox genes

Until recently, there were no genetic tools available to study sticklebacks.

However, a large number of microsatellite markers distributed across their genomes have been ordered into a genetic linkage map.

This map reveals a number of phenotypic differences in stickleback populations, some of which are markedly different from each other.

These differences are attributed to natural adaptation to freshwater conditions.

For example, genetic studies have revealed the genetic basis for two adaptive morphological traits.

These traits are pelvic reduction and lengthening of the spine.

These adaptive traits were previously thought to have no genetic basis.

However, these traits were found to be regulated by local gene expression. This research is the first to describe a new mechanism for adaptation in sticklebacks.

This genetic approach also allowed researchers to identify the genomic regions responsible for natural phenotypic variation.

These genomic regions include the LG12 chromosome region that determines sex.

The LG12 region segregates from Alaskan parents and is also different from other known chromosome regions. The LG12 region accounted for nearly one-third of the plate number variance.

In the freshwater species, a reduction in the number of lateral plates was observed. This change occurred within a few decades.

In addition, the genetic mutation responsible for pelvic reduction occurs in a common DNA region in stickleback populations throughout the world.

This suggests that both nine spine and three spine sticklebacks might have inherited this same sex-determining mechanism from a common ancestor.

In addition to revealing the genetic basis for natural phenotypic variation, this study also demonstrated the use of transgenic approaches.

Transgenic approaches are used to identify genes in specific locations and to identify cis-regulatory elements.

These approaches are helpful in identifying genes in natural variation and determining how they control sex.

Pel enhancer function

Regulatory changes to the development control genes have been often proposed as an evolutionary basis for morphological evolution.

However, few examples of regulatory evolution have been traced to particular sequences or to molecular signatures of selection in natural populations.

Several genetic studies have suggested that the same gene is involved in the acquisition of traits in sticklebacks, but they have not identified the specific sequences involved in the process.

One such gene, Pitx1, has been implicated in the loss of hind fins in stickleback fish.

The coding region of Pitx1 is conserved across teleosts, and it has multiple predicted transcription factor binding sites.

However, in the pelvic region, its expression is restricted. Several deletions in the Pel-501-bp enhancer region of Pitx1 have been identified in various freshwater stickleback populations.

Pelc-reduced populations have unusual morphological features.

These include pelvic hind fin loss, a loss of paired pelvic spines, and loss of the forward-most plates, which support the first dorsal spine.

These changes in pelvic morphology and function have been traced to regulatory mutations.

Pelc-reduced populations also have reduced heterozygosity near the Pel enhancer region. This heterozygosity reduction is unique to pelvic-complete freshwater populations.

It may reflect the presence of structural features in the Pel enhancer region, such as double-strand breaks and TG-dinucleotide repeats.

Pel is a putative 2.5-kb enhancer that promotes Pitx1 expression in the developing pelvis. In vivo, Pitx1 enhancer sequences increase double-strand breaks and mutability.

The extent of these breaks depends on the direction of DNA replication.

This enhancer mutability is influenced by the TG-dinucleotide-repeat sequences.

Deletions in the Pel enhancer region are also present in other freshwater stickleback populations.

A large genotyping survey revealed nine different haplotypes with staggered deletions.

Pitx1 locus

Despite its ubiquity, the Pitx1 locus is poorly characterized. In fact, it is a candidate gene for the loss of pelvic structure in multiple stickleback species.

We show that the Pitx1 locus is an exceptionally flexible locus and may be vulnerable to deletion or NHEJ repair.

This may contribute to the high prevalence of independent deletion mutations in pelvic-reduced stickleback populations.

In addition, we identify a 2.5 kb putative enhancer, Pel, located upstream of the Pitx1 locus. Pel successfully restores pelvic expression in reduced pelvic fishes.

It is driven by the drhsp70 promoter.

Its function is largely unknown, but it may have a role in neurotransmitter metabolism and hormone binding.

Using high-resolution mapping, we identified a 124 kb region of interest. A number of deletions and repeats were detected within this region.

These deletions may have occurred by re-ligation of chromosome ends or nonhomologous end joining (NHEJ).

Our results indicate that Pel is a tissue-specific enhancer and is responsible for the development of pelvic structures in derived populations.

Pel is driven by a drhsp70 promoter, which drives expression at different times and places.

Pel is also a potential candidate for the regulatory evolution of pelvic reduction in sticklebacks.

Pel is located in a noncoding region upstream of the Pitx1 locus.

Our results suggest that this region may contain ancestral enhancers.

The Ensembl pipeline identified 20,787 protein-coding genes, and 5,981 genes were identified as lineage-specific gene expansions.

These are a sign of complex orthology relationships between sticklebacks and other species.

We also performed a DNA flexibility analysis on the Pitx1 locus. The median flexibility score for this locus was 265 kb/bp.

These values represent the tail of the flexibility scale.

Hox expression patterns in the developing pelvis

Using genomic tools, genetic studies have shed light on the mechanisms underlying the evolutionary morphology of sticklebacks.

The results have revealed the molecular basis of four longstanding evolutionary questions.

Hox genes are known to play a central role in many morphological processes in vertebrates.

They control the identity of structures in a repeating series, such as digits and somites.

Several Hox genes are expressed in the dorsal spines of sticklebacks.

Regulatory changes in coding regions of the HOXD11B locus might have contributed to the distinctive patterns of spine number and length in these fish.

During the divergence of the Apelles and Gasterosteus lineages, changes in these coding regions are likely under positive selection.

The HOXDB locus in the Gasterosteus lineage consists of three genes.

These genes are cis-regulatory elements that have been implicated in the evolution of prominent dorsal spines in this lineage.

The alleles of the three genes vary in their expression levels across spines.

HOXD11B alleles show a strong expression in the posterior regions of the dorsal spine, suggesting that the allele is involved in early development.

The high-spine allele is also associated with the addition of an additional dorsal spine in this lineage.

In addition to the HOXDB locus, two other loci play a role in the plate number.

These loci are associated with a large proportion of the variance in pelvic phenotype.

Using genetic mapping, we have revealed a near-Mendelian locus for pelvic reduction.

The locus maps to the end of linkage group 7 and accounts for about 70% of the variance in pelvic phenotype. The four other loci account for the remaining percentage.

This suggests that the locus for pelvic reduction may be a genetic constraint on pelvic reduction.

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