Cryptic diversity in Astyanax (Characiformes: Acestrorhamphidae) from the Magdalena basin, Colombia: Insights from molecular and morphometric evidence

By integrating microsatellite genotyping, phylogenetic analyses, and geometric morphometrics, this study reveals that *Astyanax* sp. in Colombia's Magdalena basin comprises two distinct cryptic lineages exhibiting significant genetic and phenotypic differentiation, thereby highlighting the need for targeted conservation strategies amidst hydroelectric threats.

The Gist

Imagine the Magdalena River in Colombia as a massive, bustling highway for fish. For decades, scientists thought a specific type of fish, the Astyanax (a small, colorful tetra), was just one big, uniform family traveling this highway. They were so common they were like the "cattle" of the river, feeding everything from birds to bigger fish.

But this new study is like putting on a pair of high-tech detective glasses and realizing: "Wait a minute. These aren't just one family. There are actually secret, distinct clans living here, and they've been hiding in plain sight."

Here is the story of what the researchers found, broken down simply:

1. The Great Dam Barrier

Imagine the Ituango Dam as a massive wall built right in the middle of the river highway. Before this wall, fish could swim freely from the top to the bottom. Now, the river is split. The researchers wanted to know: Did this wall separate the fish into different groups? Are they evolving differently on either side?

2. The Genetic "Fingerprints"

To solve this mystery, the scientists didn't just look at the fish; they looked at their DNA.

• The Microsatellite Test: Think of this as checking the fish's unique barcodes. The researchers created 17 new "barcode scanners" specifically for these fish.
• The Discovery: The barcodes revealed two distinct groups (let's call them Clan Blue and Clan Orange).
  • Clan Blue lives mostly above the dam.
  • Clan Orange lives mostly below the dam.
  • They are so different genetically that they are like cousins who haven't spoken in a long time, even though they look very similar at first glance.

3. The "Ghost" Ancestor

Here is where it gets really weird and fascinating.
When the scientists looked at the DNA of Clan Blue, they expected it to be related to other fish in the same river. Instead, they found a surprise: Clan Blue is genetically mixed up with a fish called Astyanax microlepis (found in the upper river) and even a fish called A. viejita from a completely different river system in Venezuela (the Maracaibo basin).

The Analogy: Imagine you are looking at a family photo album. You expect to see your brother and sister. Instead, you find your brother standing next to a stranger from a different country, and they look like they share a secret family recipe. It suggests that at some point in history, these different groups of fish met, mixed their genes, and then drifted apart again, leaving a confusing genetic trail.

4. The Shape-Shifting Mystery

Since the DNA was confusing, the scientists decided to look at the fish's bodies (morphology). They took high-resolution photos and used a computer to measure every curve of their bodies, like a 3D scanner.

• The Result: Even though the DNA was messy, the bodies told a clear story.
  • Clan Blue has a slightly different body shape than Clan Orange.
  • Clan Blue is a bit more "stocky" with a shorter head.
  • Clan Orange is more "streamlined" and sleek, like a speedboat.
  • The computer was able to tell them apart with 95% accuracy just by looking at their shapes.

This is like having two twins who wear the same clothes (DNA looks similar in some ways) but have slightly different builds (one is a runner, one is a swimmer). They are distinct enough to be identified, even if their family tree is tangled.

5. Why Does This Matter?

You might ask, "So what? They're just fish."

• The "Inbreeding" Problem: The study found that both groups are suffering from inbreeding (like a small town where everyone marries their cousin). This is bad news because it makes them weaker and less able to fight disease. The dam has likely made this worse by trapping them in smaller areas.
• The "Bottleneck" Effect: The researchers found evidence that these fish populations recently crashed in numbers (a "bottleneck") and are now trying to recover.
• Conservation: If we think there is only one type of fish, we might protect them as a single unit. But if there are actually two distinct species (or "cryptic species") that are genetically unique, we need to protect them separately. If we lose the "Orange Clan" below the dam, we lose a unique piece of evolution that can't be replaced by the "Blue Clan" above it.

The Big Takeaway

This paper is a story of hidden diversity. It teaches us that nature is full of secrets. Just because two fish look alike doesn't mean they are the same. By using a mix of DNA barcodes and 3D body scanning, the scientists proved that the Astyanax fish in Colombia are actually a complex family of distinct lineages.

The Lesson: We need to stop treating these fish as a single, generic group. To save them, we need to understand their unique histories, protect their specific homes, and realize that the dam has created a genetic divide that needs careful management. It's a reminder that even in a river teeming with life, there are still many mysteries waiting to be solved.

Technical Summary

1. Problem Statement

The genus Astyanax is one of the most species-rich and ecologically significant groups of Neotropical freshwater fish, yet it suffers from severe taxonomic uncertainty. Specifically, populations in the Magdalena basin of Colombia, historically identified as Astyanax fasciatus, are now taxonomically ambiguous following the reallocation of A. fasciatus to the genus Psalidodon (restricted to Brazilian basins).

• Ecological Context: These fish are abundant, play a crucial trophic role, and face severe threats from habitat fragmentation, particularly the construction of the Ituango Dam on the Cauca River.
• Knowledge Gap: There is a lack of understanding regarding the microevolutionary genetics, population structure, and potential cryptic diversity within these Colombian populations. Traditional morphological identification is unreliable due to high phenotypic plasticity, and previous genetic studies in the region have been limited.

2. Methodology

The study employed a multidisciplinary approach integrating population genetics, phylogenetics, and geometric morphometrics to resolve the taxonomic status and diversity of Astyanax sp. in the Cauca River.

• Sampling: 324 tissue samples were collected from eight sectors along the middle and lower Cauca River (2019–2021), spanning areas upstream, within the reservoir, and downstream of the Ituango Dam.
• Microsatellite Development: Researchers developed 42 novel species-specific microsatellite loci (tri-, tetra-, and pentanucleotide motifs) using Illumina MiSeq sequencing. 17 loci were polymorphic, with 14 selected for population analysis.
• Population Genetics:
  • Structure: Bayesian clustering (Structure v2.3.4) to identify genetic stocks.
  • Differentiation: AMOVA, pairwise $F_{ST}$, and Jost's $D_{EST}$.
  • Diversity Metrics: Allelic richness ($A_r$), observed/expected heterozygosity ($H_O$, $H_E$), inbreeding coefficients ($F_{IS}$), and effective population size ($N_e$).
  • Demography: Tests for recent bottlenecks (BOTTLENECK software) and M-ratio analysis.
• Phylogenetics: Sequencing of mitochondrial (COI) and nuclear (rag2) genes. Analyses included Bayesian Inference (MrBayes) and Maximum Likelihood (IQ-TREE) to reconstruct relationships with congeners (including A. microlepis, A. viejita, and Psalidodon).
• Geometric Morphometrics: 138 specimens were photographed, and 9 landmarks were digitized. Generalized Procrustes Analysis (GPA) was used to remove size/position effects. Linear Discriminant Analysis (LDA) and Thin-Plate Spline (TPS) deformation grids were used to quantify body shape variation and test for morphological distinctness.

3. Key Results

A. Genetic Structure and Diversity

• Two Distinct Lineages: Bayesian clustering ($K=2$) revealed two distinct genetic stocks:
  • Asp1: Predominant in upstream and dam-proximate sectors (S1–S4, PHI).
  • Asp2: Predominant in downstream sectors (S5–S8).
• Genetic Differentiation: Significant differentiation was found between upstream/dam-proximate and downstream sectors ($F_{ST} > 0.12$, $D_{EST} > 0.08$).
• Diversity & Demography: Both stocks exhibited high genetic diversity ($H_E \approx 0.86$) but also significant inbreeding ($F_{IS} > 0.10$), with higher inbreeding in the downstream stock (Asp2).
• Bottlenecks: Evidence of recent bottleneck events was detected across all sectors. However, effective population sizes ($N_e$) remained high (Global $N_e > 1000$), suggesting large reproductive pools despite fragmentation.

B. Phylogenetic Relationships

• Lineage 1 (Blue Clade): Comprised Asp1 samples, A. microlepis (from the upper Cauca), and A. viejita (from the Maracaibo basin, Venezuela). This lineage is distinct from Psalidodon.
• Lineage 2 (Orange Clade): Comprised Asp2 samples and Astyanax sp. from the main Magdalena River channel.
• Taxonomic Implication: The study unequivocally dismisses the identification of these populations as Psalidodon fasciatus.

C. Morphometric Differentiation

• Distinct Morphotypes: Linear Discriminant Analysis (LDA) separated three groups with 95.65% classification accuracy:
  1. A. microlepis: Robust body profile.
  2. Asp1: Intermediate body shape, shorter head, posterior adipose fin, and ventral fin insertions.
  3. Asp2: Streamlined morphology.
• Phenotypic vs. Genotypic Discordance:
  • Asp1 vs. A. microlepis: Genetically similar (sharing a mitochondrial lineage) but morphologically distinct.
  • Asp1 vs. Asp2: Genetically distinct (different nuclear and mitochondrial lineages) but morphologically similar (cryptic species), differing only in subtle fin placements and head proportions.

4. Key Contributions

1. Novel Molecular Tools: Successfully developed and validated the first set of species-specific microsatellite loci for Astyanax in Colombia, enabling high-resolution population monitoring.
2. Discovery of Cryptic Diversity: Confirmed the existence of two distinct evolutionary lineages (Asp1 and Asp2) within the "Astyanax sp." complex of the Cauca River, which were previously indistinguishable morphologically.
3. Resolution of Taxonomic Limbo: Provided molecular evidence that the Colombian populations are not Psalidodon fasciatus but represent distinct lineages within Astyanax (sensu lato), one of which shares a mitochondrial history with A. microlepis and A. viejita.
4. Insight into Evolutionary Mechanisms:
   • Suggested introgression or incomplete lineage sorting explains the shared mitochondrial DNA between morphologically distinct A. microlepis and Asp1.
   • Proposed stabilizing selection or niche conservatism explains the morphological similarity between the genetically divergent Asp1 and Asp2 (cryptic species).

5. Significance

• Conservation Management: The identification of two distinct genetic stocks (Asp1 and Asp2) separated by the Ituango Dam is critical for conservation. The downstream stock (Asp2) shows higher inbreeding, indicating it may be more vulnerable to the dam's isolation effects.
• Systematic Revision: The findings necessitate a taxonomic revision of Astyanax in the Magdalena and Maracaibo basins, moving away from the misapplied P. fasciatus label.
• Ecological Understanding: The study highlights how hydroelectric infrastructure impacts microevolutionary processes, creating genetic discontinuities in highly abundant species.
• Evolutionary Biology: The case of Asp1 and A. microlepis serves as a compelling example of how mitochondrial introgression can mask deep phenotypic divergence, while the Asp1/Asp2 pair illustrates how strong stabilizing selection can maintain morphological stasis despite genetic divergence.

In conclusion, this study provides a robust framework for understanding the complex biodiversity of trans-Andean Astyanax, emphasizing the need for integrated molecular and morphological approaches to guide conservation strategies in heavily modified Neotropical river systems.