Extra lobes on morning glory leaves point to a more complex genetic story

Ivyleaf morning glory has a well-studied genetic distinction between heart-shaped leaves with smooth margins and leaves with three lobes. But some plants produce an additional variation: leaves with four or five lobes. Research from the University of Toronto suggests that this extra-lobing does not follow the same kind of simple inheritance pattern.

The study, posted on bioRxiv on November 2, 2026, links the tendency to produce extra lobes to two genomic regions. Those regions contain genes whose counterparts in other plants participate in hormone responses and leaf development. The work is a preprint that has not been peer reviewed, and it identifies genetic associations rather than demonstrating which genes cause the additional lobes.

Testing inheritance in a greenhouse

Martin R. Henry and colleagues crossed a three-lobed plant from a Delaware line with a plant from a Maryland line that regularly produced five-lobed leaves. Both parental lines had undergone three generations of self-fertilization. A single first-generation offspring then self-fertilized to produce the second-generation seeds used in the experiment.

The team planted 500 second-generation individuals and assessed their leaves after one month. Classification depended on the leaves a plant produced, not on every leaf having an identical shape. A plant could, for example, enter the extra-lobed category because it had at least one four-lobed leaf.

The reported counts were 184 plants with only three-lobed leaves, 173 producing four-lobed leaves but no five-lobed leaves, 131 producing both four- and five-lobed leaves, and one producing five-lobed leaves without four-lobed leaves. These counts total 489 rather than the 500 planted; the manuscript does not explain that difference.

The distribution differed strongly from both a 3:1 inheritance expectation and a 1:2:1 expectation, with reported P values below 0.0001. These tests argue against the particular simple single-locus models examined. They do not exclude every possible one- or two-locus explanation. The researchers considered a two-locus 9:3:3:1 test but did not conduct it because only one plant fell into the final category.

Two regions associated with extra lobes

For the genomic analysis, the researchers combined equal amounts of DNA from 71 three-lobed plants into one pool and from 71 extra-lobed plants into another. This approach, called bulked segregant analysis, looks for genetic variants that occur at different frequencies between groups with contrasting traits.

The comparison identified associated regions on chromosomes 2 and 4, spanning approximately 1.65 million and 860,000 DNA bases, respectively. Such regions, called quantitative trait loci, can contain multiple genes. Their detection supports a genetic contribution to extra-lobing but does not establish the number or identity of causal variants.

The chromosome 2 finding also connects with earlier research on the species’ main leaf-shape distinction. That work identified a 117,000-base insertion-deletion associated with heart-shaped versus three-lobed leaves. The newly mapped region overlaps about 52% of that interval. Both parents in the present cross were lobed and retained the interval, so the overlap does not show that the same presence-or-absence change explains extra lobes.

Candidate genes, not confirmed mechanisms

Across the two regions, the team identified 98 morning glory gene IDs. Comparisons with Arabidopsis and tomato annotations highlighted candidates related to auxin, a plant hormone involved in growth, and to other aspects of leaf development.

Chromosome 2 candidates included counterparts of PIN-LIKES 1, AUXIN RESPONSE FACTOR 8 and KANADI 1. Their known functions in other plants make them plausible leads involving auxin handling, hormone responses and the establishment of leaf surfaces. Chromosome 4 contained candidates related to organ-size regulation and developmental gene activity.

These are hypotheses based on genomic location and comparisons across species. The study did not manipulate the candidate genes or directly measure auxin distribution. Its functional-enrichment analysis also produced leading categories whose relationship to leaf shape was unclear, as the authors acknowledge.

Other limits concern where and when the findings apply. The experiment used one cross, descendants of a single first-generation plant, and one greenhouse setting. Growing randomized plants in a common environment helps reduce environmental confounding, but it cannot rule out environmental effects or interactions between genes and growing conditions. Scoring at one month also leaves later leaf production uncharacterized.

The ecological question remains open. The study did not test whether extra lobes alter temperature regulation, water use, herbivory or reproductive success. Its contribution is a genetic starting point: two associated regions and a set of candidates, rather than a confirmed developmental mechanism or an explanation of why extra-lobed leaves persist.