Secondary phloem diversity and evolution in Bignonieae (Bignoniaceae)

Area of Science:

• Plant anatomy
• Evolutionary biology
• Systematics

Background:

• Phloem evolution studies typically focus on broad scales (e.g., entire vascular plants or major groups).
• Evolutionary shifts in phloem diversification within single lineages remain underexplored.
• The Bignonieae tribe (Bignoniaceae) offers a unique model due to its diverse and conspicuous phloem.

Purpose of the Study:

• To investigate detailed patterns of phloem evolution within the Bignonieae tribe.
• To elucidate evolutionary shifts and correlations in phloem anatomical traits.

Main Methods:

• Coded 19 phloem characters across 56 species representing all 21 genera of Bignonieae.
• Reconstructed ancestral states using maximum-likelihood on a time-calibrated molecular phylogeny.
• Tested for directional evolution and correlations among discrete phloem traits.

Main Results:

• Observed significant diversity in phloem features, generally conserved within smaller clades.
• Identified contrasting phloem patterns between major lineages (e.g., fibrous vs. scarcely fibrous).
• Found directional evolution in fibre abundance and sieve area number in specific clades, but not overall for Bignonieae. Positive correlations exist between sieve element length and sieve plate/storied structure, and between fibre abundance and multiple other traits.

Conclusions:

• Secondary phloem in Bignonieae is highly diverse, with sister lineages showing distinct anatomies driven by fibre abundance evolution.
• Fibre abundance diversification is correlated with sieve tube arrangement, morphology, companion cell number, and parenchyma type.
• Findings challenge existing hypotheses on phloem cell evolution and highlight the need for studies at both narrow and broad taxonomic scales.