Why six and seven are implausible ancestral angiosperm chromosome numbers

Area of Science:

• Plant evolutionary biology
• Genomics
• Cytogenetics

Background:

• Historically, ancestral plant chromosome numbers were inferred from meiotic counts in extant species, leading to a hypothetical base number.
• Since 2017, comparative genomics using chromosome-level genome assemblies offers an alternative approach.
• These two methods have produced conflicting ancestral base numbers for flowering plants (n=7 vs. n=16).

Purpose of the Study:

• To review how traditional assumptions about chromosome number evolution have influenced inferences of ancestral numbers in plants.
• To highlight the discrepancies between historical and modern genomic approaches to inferring plant chromosome numbers.

Main Methods:

• Review of historical botanical literature and benchmark papers (e.g., G. L. Stebbins).
• Analysis of model-based chromosome number evolution studies.
• Comparison of inferred ancestral numbers from meiotic counts versus comparative genomics of syntenic blocks.

Main Results:

• Traditional methods and some model-based studies perpetuate biases by favoring low ancestral chromosome numbers.
• This bias may explain the discrepancy between low inferred numbers (n=7) and higher numbers (n=16) from comparative genomics.
• Recent data, including from early-diverging lineages, contradicts the assumed low-to-high trend.

Conclusions:

• The long-assumed trend of increasing chromosome numbers from low to high across flowering plant evolution is not supported by current data.
• Newer genomic approaches provide more robust insights into plant chromosome number evolution.
• Re-evaluation of historical assumptions is crucial for accurate phylogenetic inference.