Cytonuclear Conflict and Reticulate Evolution in the Morelloid Clade (Solanum, Solanaceae): Insights from Genome Skimming and Network Phylogenomics

By integrating genome skimming with network phylogenomics, this study reveals that pervasive incongruence between nuclear and plastid trees in the Morelloid clade is driven primarily by repeated chloroplast capture and reticulate evolution rather than incomplete lineage sorting.

The Gist

Imagine the Morelloid clade (a group of plants often called "black nightshades") as a massive, chaotic family reunion. Some of these plants are annoying weeds, but others are important food crops in Africa and South America. They are cousins to the tomato and potato, holding secret genetic recipes that could help us grow better crops in the future.

However, trying to draw a family tree for these plants has been a nightmare. For years, scientists tried to map their history using a simple "branching tree" model (like a standard family tree where you have one mom and one dad, and your kids branch off). But the Morelloids don't follow the rules. They are messy, they mix, and they swap parts of their DNA like trading cards.

Here is what this new study discovered, explained simply:

1. The "Two-Story" Mystery

Every plant has two main instruction manuals:

• The Nuclear Manual (The Big Book): This contains the main instructions for the plant's body, inherited from both parents.
• The Chloroplast Manual (The Pocket Guide): This is a smaller, specialized manual for making energy (photosynthesis), passed down only from the mother plant.

Usually, these two manuals tell the same story about who the plant's ancestors were. But in the Morelloid family, the two manuals tell completely different stories.

2. The Great DNA Swap (Chloroplast Capture)

The researchers used a high-tech method called Genome Skimming. Think of this like taking a high-speed photo of a plant's entire genetic library to get a quick, complete snapshot of its DNA.

They found that the Morelloids have been playing a game of "genetic tag."

• The Analogy: Imagine a group of friends (different plant species) hanging out. One friend (Species A) has a cool jacket (a specific Chloroplast). Another friend (Species B) really likes that jacket. They hang out, have a baby, and the baby wears the cool jacket from Species A, but the rest of their body looks like Species B.
• The Science: This is called Chloroplast Capture. The plants hybridize (mix), and then the "baby" plants keep breeding back with one of the parent groups. Over time, the whole group ends up looking like one species genetically, but they are all wearing the "jacket" (chloroplast) of a completely different species.

3. The "Reticulate" Web

Because of all this swapping, you can't draw a straight tree. You have to draw a web or a net (which scientists call "reticulate evolution").

• The Analogy: A standard family tree is like a river flowing downstream, splitting into smaller streams. A Morelloid family tree is like a spaghetti bowl. The strands are all tangled up, crossing over each other, and merging in ways that make it impossible to say, "This branch came only from that branch."

4. The African and American Mix-Up

The study focused heavily on two main groups:

• The Pan-American Diploids: These are the "standard" 2-set chromosome plants found in the Americas (like Solanum americanum).
• The African Polyploids: These are the "super-charged" plants in Africa with multiple sets of chromosomes (like Solanum scabrum).

The researchers found that the African plants didn't just evolve in isolation. They seem to have "stolen" their chloroplasts (their maternal lineages) from the American plants through ancient hybridization events. It's like an African plant saying, "I look like an American plant on the outside, but my internal energy engine is actually from a different American cousin."

5. Why This Matters

For a long time, scientists thought these plants were just "messy weeds" with confusing names. This study proves they are evolutionary masters of disguise.

• The "Unstable" Polyploids: The study found some plants that are "ephemeral polyploids"—essentially genetic experiments that formed but haven't fully settled into a stable species yet. They are a mix of everything, like a smoothie that hasn't been blended perfectly.
• Crop Improvement: Because these plants are so good at mixing and matching genes, they are a goldmine for farmers. If we understand how they swap genes, we might be able to take their "superpowers" (like resistance to disease) and put them into our potatoes and tomatoes.

The Bottom Line

The Morelloid clade isn't a neat family tree; it's a family reunion where everyone is swapping clothes and telling different stories about their grandparents.

By using advanced DNA scanning, this paper finally untangled the spaghetti. It showed us that hybridization and gene-swapping are the main engines driving the evolution of these plants, not just simple branching. This "messy" history is actually what makes them so diverse and potentially so useful for feeding the world.

Technical Summary

1. Problem Statement

The Morelloid clade (black nightshades) within the genus Solanum comprises approximately 76–78 globally distributed species, including economically important orphan crops and reservoirs of genetic diversity for potato and tomato breeding. Despite their importance, the evolutionary history of this clade remains poorly understood due to:

• Reticulate Evolution: The group contains numerous polyploid species with unknown parental origins, suggesting complex histories of hybridization, introgression, and backcrossing.
• Morphological Plasticity: High phenotypic variability and overlapping species boundaries make morphological identification unreliable.
• Methodological Limitations: Previous phylogenetic studies relied on limited loci or single-marker datasets, which are insufficient to disentangle incomplete lineage sorting (ILS) from hybridization/introgression.
• Cytonuclear Discordance: Preliminary data suggested conflicting signals between nuclear and plastid (chloroplast) genomes, but the mechanisms driving this discordance (e.g., ILS vs. chloroplast capture) were unresolved.

2. Methodology

The authors employed a phylogenomic approach combining deep genome skimming with advanced network analyses to resolve these conflicts.

• Taxon Sampling: The study analyzed 99 accessions representing 25 Morelloid species. This included 28 newly extracted sequences from herbarium specimens (using historical DNA protocols) and 71 sequences from published datasets (including re-analyzed data from Lin et al., 2022, and Gagnon et al., 2022).
• Data Generation & Assembly:
  • Genome Skimming: Generated ~10 Gb of Illumina NovaSeq X Plus data per sample.
  • Plastomes: Assembled 93 new complete circular plastomes using GetOrganelle, GeneMiner2, and reference-guided methods.
  • Nuclear Markers: Recovered nuclear genes from the Angiosperms353 and Conserved Ortholog Set (COS) gene families using HybPiper.
• Phylogenetic Analyses:
  • Plastid Phylogeny: Constructed a Maximum Likelihood (ML) tree using complete plastomes (including indel coding) via IQ-TREE.
  • Nuclear Phylogeny: Generated coalescent-based species trees using ASTRAL from individual gene trees derived from concatenated a353, COS, and combined datasets.
  • Network Analysis: Used SplitsTree (NeighborNet) to visualize non-tree-like signals and calculated delta scores to quantify tree-likeness.
  • Conflict Quantification: Calculated Gene Concordance Factors (gCF) and Site Concordance Factors (sCF) to measure support and conflict.
• Discordance Testing:
  • Multispecies Coalescent (MSC) Simulations: Used DendroPy to simulate 10,000 gene trees under a neutral ILS model to establish a null distribution of Robinson-Foulds (RF) distances.
  • Hypothesis Testing: Compared observed RF distances between empirical plastid and nuclear trees against the simulated ILS distribution. Branches recovered in ≤1% of simulations were interpreted as evidence of chloroplast capture rather than ILS.
  • Fused Trees: Utilized Phylofusion to visualize rooted networks and reticulate arches indicating introgression.

3. Key Results

A. Plastid Phylogeny

The plastid tree resolved three major clades with high support:

1. Clade I: Pan-American diploids (primarily S. americanum and S. nigrescens).
2. Clade II: African polyploids (including S. scabrum, S. nigrum, and the S. tarderemotum complex).
3. Clade III: Ancestral diploid lineages (S. sarrachoides, S. nitidibaccatum, and specific S. americanum accessions).

• Anomalies: Two S. nigrum accessions from China were found in different clades (one in Clade I, one in Clade II), suggesting cryptic diversity or misidentification.

B. Nuclear Phylogeny & Network

• The nuclear species tree showed different topological arrangements compared to the plastid tree.
• Low Concordance: Gene concordance factors (gCF) were generally low (0–25%), while site concordance factors (sCF) were higher (25–50%), indicating that individual sites support branches more strongly than individual gene trees do. This suggests extensive gene tree conflict.
• Network Structure: The NeighborNet network revealed a "web" rather than a bifurcating tree, particularly within the S. tarderemotum complex and between S. americanum and S. nigrescens. Delta scores indicated moderate tree-likeness, with Clade IV (African polyploids) being the least tree-like.

C. Evidence for Reticulate Evolution

• Rejection of ILS: The observed RF distance between the empirical plastid and nuclear trees (130) was significantly lower than the null distribution expected under ILS (peak ~145).
• Chloroplast Capture: Approximately 40% of plastome nodes showed <1% recovery in MSC simulations. These low-frequency conflicts were concentrated in Clades I and II, providing strong statistical evidence that repeated chloroplast capture (introgression of plastids across species boundaries) is the primary driver of cytonuclear discordance, rather than ILS.
• Specific Introgression Events:
  • Pan-American Lineages: Extensive admixture between S. americanum and S. nigrescens.
  • African Polyploids: The S. tarderemotum complex appears to be an allotetraploid hybrid involving a maternal lineage related to S. hirtulum (evidenced by plastome) and a paternal lineage related to S. memphiticum (evidenced by nuclear data).
  • Chinese S. nigrum: One accession grouped with African polyploids, while another grouped with Pan-American diploids, indicating distinct evolutionary histories or cryptic species.

4. Key Contributions

1. First Genome-Skimming Phylogeny: This is the first study to utilize complete plastomes and hundreds of nuclear loci from genome skimming to resolve the Morelloid clade, overcoming the resolution limits of previous marker-based studies.
2. Disentangling ILS vs. Introgression: By combining empirical data with rigorous MSC simulations, the authors definitively demonstrated that cytonuclear discordance in this clade is driven by hybridization and chloroplast capture, not just incomplete lineage sorting.
3. Taxonomic Re-evaluation: The study re-identified several "Group 5" S. americanum accessions from previous literature as distinct species or polyploids (e.g., S. retroflexum, S. scabrum), correcting previous misidentifications.
4. Evolutionary History of African Polyploids: Provided a hypothesis for the origin of African polyploids as allopolyploids derived from hybridization events involving Pan-American diploids and African lineages, with subsequent chloroplast capture.

5. Significance

• Evolutionary Biology: The study challenges the strictly bifurcating tree model for the Morelloid clade, illustrating that reticulate evolution is pervasive. It highlights the necessity of using network-based approaches and cytonuclear comparisons to understand plant evolution in groups with high polyploidy and hybridization rates.
• Crop Improvement: By clarifying the genetic relationships and identifying the true parental origins of polyploid species, this research aids in the utilization of Morelloid species as genetic reservoirs for crop improvement (e.g., disease resistance in potato and tomato).
• Methodological Benchmark: The workflow combining herbarium genome skimming, MSC simulations, and phylogenetic networks serves as a robust template for resolving complex evolutionary histories in other taxonomically difficult plant groups.

In conclusion, the Morelloid clade is not a simple tree but a complex network shaped by repeated hybridization and chloroplast capture, particularly between Pan-American diploids and African polyploids.