The polyketide pathway in sporopollenin biosynthesis is specific to land plants (Embryophyta)
Bibliographic record
Abstract
Abstract Background and Aims Sporopollenin (SP) is a complex biopolymer in the outer wall of spores and pollen and provides protection from environmental stresses. Its extraordinary chemical resistance, especially to acetolysis, was widely used to identify SP in biological specimens. This broad definition of SP led to claims for its widespread occurrence among diverse embryophyte and non-embryophyte taxa. We previously proposed a biochemical definition that can be used to distinguish genuine SP from other chemically resistant cell wall materials. The definition was centred on ASCL (Anther-Specific Chalcone synthase-Like), an embryophyte-specific enzyme of the polyketide pathway that provides precursors for SP biosynthesis. Herein, we examine the evolution and distribution of all five enzymes (CYP703A, CYP704B, ACOS, ASCL and TKPR) of the polyketide pathway and propose a new, more comprehensive definition of SP. Methods We performed BLASTp searches, phylogenetic tree construction, protein modeling and sequence analysis to determine the presence or absence of ACOS and TKPR in embryophytes and streptophytic algae. Key Results We found evidence that all five enzymes of the polyketide pathway evolved from ancestral enzymes of primary metabolism and ACOS, ASCL and TKPR were co-selected during evolution. The dosage of all five genes has been subjected to strict evolutionary control and, in some taxa, synteny has provided a selective advantage. All five enzymes are present in embryophytes but absent in green algae, indicating that the polyketide pathway and therefore SP is embryophyte-specific. Conclusions The addition of the polyketide pathway in the definition of genuine SP will allow separation of SP from algaenans and other chemically resistant ‘SP-like’ algal spore wall substances. This study further signifies SP as an evolutionary innovation unique to the embryophyte lineage and encourages research on possible evolutionary relationship between algal spore wall ‘SP-like materials’ and embryophyte SP.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".