A long-standing mystery about plant evolution is finally being solved. Scientists have used a powerful imaging method to track how chromosome numbers changed in Medicago. This group of plants includes alfalfa, a vital crop for animal feed.
The new findings suggest that Medicago species with seven chromosomes did not become more complex over time. Instead, they likely evolved by losing and rearranging genetic material from an ancestor that had eight chromosomes. This process was not a simple fusion of chromosomes. It involved extensive breaking and reassembly across three ancestral chromosomes.
The study also shows how stable chromosomes are within the Medicago sativa group. This helps explain the connections between genome structure, how new species form, and how crops evolve.
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Start Your News DetoxUnraveling Chromosome Evolution
Changes in chromosome numbers are common in plants. But figuring out which way these changes happened is often hard. This is because the original ancestral species are usually extinct or unknown. For Medicago, this question has been debated for years. Scientists wondered if seven-chromosome species came from eight-chromosome ones, or the other way around.
Even with advances in genome sequencing, detailed chromosome-level evidence is still limited. This is especially true for plants with many repetitive DNA sections and complex genomes. Standard methods often lack the detail needed to follow entire chromosomes across related species. These challenges made it hard to map the evolutionary path of chromosome changes in Medicago.
Researchers from Shihezi University, Northwest A&F University, and Anhui Normal University found a solution. They reported in Horticulture Research that chromosome-specific painting can clarify the evolutionary history of chromosome changes in Medicago. Their work suggests that species with seven chromosomes came from an eight-chromosome ancestor. This happened through complex breaking and joining events. The study also confirmed strong chromosome stability among major alfalfa relatives.
Mapping Chromosome Changes
The team created a whole-chromosome painting system for alfalfa's eight chromosomes. They used over 736,000 chromosome-specific DNA probes. Then, they used a technique called sequential fluorescence in situ hybridization. This allowed them to compare many Medicago samples. This method let them directly see chromosomes and compare species with eight and seven chromosomes.
Within the M. sativa group, chromosome structures were very stable. This was true for both diploid and autotetraploid forms. No rearrangements between chromosomes were found. However, M. polymorpha showed a different pattern. Earlier ideas suggested its reduced chromosome number came from a simple fusion of chromosomes three and seven. But the new evidence shows that chromosomes three, five, and six broke into pieces. These pieces were then reorganized into two new chromosomes.
Molecular Evidence Supports a Descending Model
The researchers also looked at ribosomal DNA patterns. In species with eight chromosomes, 5S rDNA is on chromosomes two and four. The 45S rDNA is on chromosome six. In M. polymorpha with seven chromosomes, 5S rDNA is only on chromosome two. The 45S rDNA moves to chromosome four.
These patterns, along with the structural findings, strongly support the idea of a reduction from eight to seven chromosomes. Phylogenetic analysis suggests this change happened about 12 million years ago. The doubling of chromosomes in alfalfa likely occurred around 6 million years ago.
The study shows that chromosome evolution in Medicago was not a simple, single step. Instead, it involved many coordinated rearrangements. By combining chromosome observations with evolutionary analysis, the researchers provide clear evidence. This evidence shows how chromosome changes led to the diversity in this important crop group. The results also indicate that genome assemblies alone might not fully capture complex chromosome histories, especially in areas rich in repetitive DNA.
Impact on Breeding and Future Research
The chromosome-painting method developed in this study has practical uses beyond answering evolutionary questions. It can help identify chromosome differences among Medicago varieties. It can also track chromosome stability over generations and confirm chromosome inheritance in hybrids.
These applications make the technique valuable for both evolutionary research and improving crops. The study also offers a useful way to investigate chromosome evolution in other plant groups. In these groups, similar genomic changes are still not well understood.
Deep Dive & References
Chromosome-specific painting provides insights into the karyotype evolutionary direction and trajectory in the genus Medicago - Horticulture Research, 2025
