Question

In a recent study of cichlid fish inhabiting Lake Victoria in Africa, Nagl et al. (1998. Proc. Natl. Acad. Sci. IUSA/ 95: \(14,238-14,243\) ) examined suspected neutral sequence polymorphisms in noncoding genomic loci in 12 species and their putative river-living ancestors. At all loci, the same polymorphism was found in nearly all of the tested species from Lake Victoria, both lacustrine and riverine. Different polymorphisms at these loci were found in cichlids at other African lakes. (a) Why would you suspect neutral sequences to be located in noncoding genomic regions? (b) What conclusions can be drawn from these polymorphism data in terms of cichlid ancestry in these lakes?

Short answer

Answer: Neutral sequences are suspected to be located in noncoding genomic regions because they do not affect an organism's phenotype or fitness, and mutations in these regions do not result in significant alterations to the organism's functional proteins. Polymorphism data suggests that cichlid populations in Lake Victoria share a common ancestry and genetically diverged from cichlid populations in other African lakes. The presence of similar polymorphisms in both lacustrine and riverine cichlid species from Lake Victoria indicates they likely descended from a single ancestral population, while different polymorphisms in cichlids from other African lakes indicate distinct ancestral lineages that evolved separately.

Step-by-step solution

Part (a): Understanding Neutral Sequences and Noncoding Genomic Regions

Neutral sequences refer to DNA sequences that do not have any effect on an organism's phenotype or fitness. They may undergo mutations or change over time without affecting the organism's survival or ability to reproduce due to their lack of functional importance. Noncoding genomic regions are sections of DNA that do not produce proteins, meaning they are not translated into functional products like enzymes or structural proteins. Since neutral sequences do not affect an organism's phenotype or fitness, it would be reasonable to suspect that they are located within noncoding genomic regions, as any changes or mutations in these regions will not result in significant alterations to the organism's functional proteins or phenotype.

Part (b): Interpreting Polymorphism Data

The polymorphism data presented in the study reveals that the same polymorphism was found in nearly all tested species from Lake Victoria, both lacustrine (lake-dwelling) and riverine (river-dwelling), while different polymorphisms at these loci were found in cichlid species in other African lakes. This suggests that the cichlid populations in Lake Victoria share a common ancestry and genetically diverged from cichlid populations in other African lakes. Since both lacustrine and riverine cichlid species from Lake Victoria show the same polymorphisms, it implies that they likely descended from a single ancestral population that colonized the lake and then diverged into multiple species over time. The presence of different polymorphisms in cichlids from other African lakes indicates distinct ancestral lineages in those lakes, which evolved separately from the ancestral lineage in Lake Victoria.

Key concepts

Neutral Sequences

Neutral sequences are intriguing when examining DNA because they do not influence the organism's phenotype or overall fitness. This means these sequences can mutate or alter without impacting the organism's ability to survive or reproduce. The stability of these sequences is beneficial for studying evolutionary changes because they provide a kind of baseline that isn't skewed by environmental pressures.

In many cases, these neutral sequences are believed to be situated within noncoding genomic regions. This is due to their lack of direct involvement in coding for proteins. Changes in these regions are unlikely to result in any significant alterations to the organism's functions, since they aren't involved in creating the machinery or structural elements that facilitate life.

• Neutral sequences undergo changes silently, not impacting the organism.
• They provide a clear view of evolution without interference from environmental adaptations.
• These sequences are usually found in parts of the genome that do not affect the organism's phenotype.

Noncoding Genomic Regions

The human genome, like that of many species, contains vast areas known as noncoding genomic regions. These parts do not translate into proteins but can still play significant roles in the regulation and organization of the genome's function. For many scientists, these regions are like a mysterious part of DNA, leading to ongoing research about their purpose.

While these regions don't produce proteins, their role may include influencing when and if genes are expressed. They help orchestrate the timing and use of various genetic elements, acting as a kind of conductor to the genome's symphony. Importantly, in studying evolutionary processes, the presence of neutral sequences within noncoding regions can be quite enlightening.

• Noncoding regions make up a significant portion of the genome.
• They do not produce protein but can influence gene expression.
• Scientists explore these areas for insights into evolutionary history.

Cichlid Fish Ancestry

Cichlid fish, particularly those in Lake Victoria, have been a focal point for evolutionary biologists due to their diverse speciation. The studied polymorphisms, which are variations in the DNA sequences, help to trace lineage and genetic diversity within the cichlid populations. This study by Nagl et al., notes a common pattern of polymorphisms among cichlids in Lake Victoria, thereby indicating a shared ancestral origin.

Such findings emphasize that the lake-dwelling and river-dwelling cichlids likely descended from one ancestral population. This shared ancestry suggests a colonization event that led to the proliferation and diversity of cichlids within the lake, adapting to various ecological niches over time. In contrast, the differing polymorphisms found in cichlids from other African lakes imply that these lakes harbor distinct evolutionary lineages.

• Polymorphisms present a genetic breadcrumb of ancestry and evolution.
• Shared polymorphisms in Lake Victoria cichlids point to a common ancestor.
• Differing polymorphisms in other lakes suggest separate evolutionary paths.