Andromonoecy in Passiflora: Floral Development, Function, and Stylar Movement

Overview

Passiflora incarnata (purple passionflower) is a widespread native vine of the southeastern United States, well known for its complex flowers and its role as a larval host for numerous butterfly species. In addition to its ecological importance, P. incarnata displays a fascinating case of floral developmental variation known as andromonoecy — the simultaneous production of male and hermaphroditic flowers on the same individual plant.

Unlike the simple “male vs. hermaphrodite” floral system often described in Passiflora, our research has uncovered much finer-scale variation. Individual vines of P. incarnata produce at least four distinct floral morphs, representing a spectrum of gynoecial development and functional reproductive capacity. Understanding the developmental, anatomical, and functional basis for this morph variation offers novel insights into the evolution of plant reproductive systems.

Research Questions

Our ongoing research aims to address multiple interrelated questions:

• What developmental processes give rise to the multiple floral morphs observed in P. incarnata?
• How does reproductive capacity vary across morphs, particularly regarding pollen tube growth and ovule penetration?
• What anatomical changes in stylar tissue are responsible for pollen tube failure in some morphs?
• What cellular structures control the striking stylar movement that occurs during anthesis?
• How does this system of partial andromonoecy fit into the broader evolutionary pathways of floral sexual systems in angiosperms?

Discovery of Multiple Floral Morphs (Krosnick et al. 2017)

Field surveys and morphometric analysis of 233 flowers identified five floral morphs grouped into four distinct developmental classes:

• Morph 1: Fully aborted pistils (male), styles remain erect.
• Morph 2: Partially developed ovaries, erect styles (male).
• Morph 3: Further developed ovaries and partial stylar movement (intermediate).
• Morph 4a: Fully developed pistil, styles bend down to the anther level (hermaphroditic).
• Morph 4b: Fully developed pistil, styles bend further below the anther level (hermaphroditic variant).

Floral size, ovary size, style length, stigma width, and nectar production generally increase across this developmental series.

Figure 2 from Krosnick (2017) showing the variation in floral morphs observed in wild populations.

Reproductive Function Across Morphs (Ongoing)

Recent work led by undergraduate researchers is evaluating the functional reproductive capacity of each morph through controlled pollinations and histological examination:

• Controlled crosses performed on genetically distinct individuals revealed that:
  • Pollen germination occurs on the stigmas of all morphs.
  • Pollen tubes generally fail to penetrate styles or ovaries in morphs 1 and 2.
  • Morph 3 occasionally allows pollen tubes into the style but not reliably into the ovary.
  • Morph 4a consistently allows ovary penetration; morph 4b shows variable success.
• Histological cross-sections show arrested development of stylar transmitting tissue in the male morphs (1 & 2), while functional morphs exhibit fully differentiated transmitting tissue.

These results suggest that developmental arrest of the transmitting tissue within the style is a key anatomical barrier to successful fertilization in the male morphs.

• Stylar deflexion occurs within 2–3 hours after anthesis, with varying degrees of movement tied to floral morph:
  • Morphs 1 & 2: No deflexion (erect).
  • Morph 3: Partial deflexion.
  • Morphs 4a & 4b: Full deflexion.
• The cellular mechanisms behind this movement remain unknown, but are likely related to specialized tissues within the style. Analogous tissue systems in Marantaceae have been identified as thigmonastic trigger mechanisms (Pischtschan & Claßen-Bockhoff 2010; Jerominek & Claßen-Bockhoff 2015), which may provide a developmental model for future studies.

Ecological & Evolutionary Implications

The reproductive success of P. incarnata relies on efficient pollination by its primary pollinators, particularly carpenter bees (Xylocopa virginica). The ability of floral morphs to develop functional pistils — or not — directly influences both male and female reproductive fitness.

The occurrence of multiple morphs within a single clonal plant suggests plasticity in developmental regulation. This system offers a valuable natural experiment for studying evolutionary transitions between hermaphroditism, andromonoecy, and functional monoecy in flowering plants. Developmental arrest models indicate that partial suppression of gynoecium development may represent early steps along this evolutionary continuum.

Ongoing Research Directions

Current and future work includes:

• Microscopic analysis of stylar tissues using TEM to identify potential contractile or specialized cell types associated with movement.
• Expanded functional crosses to quantify seed set across morphs.
• Investigation of hormonal or resource-based regulation of morph ratios within individual vines.
• Comparative studies across other Passiflora species to evaluate how widespread this developmental system may be.

TEM image of tissues in regions where stylar movement occurs.

Sample Publications

• Krosnick, S.E., Perkin, J.S., Schroeder, T.S., Campbell, L.G., Jackson, E.B., Maynord, S.C., Waters, C.G., Mitchell, J.S. (2017). New insights into floral morph variation in Passiflora incarnata L. (Passifloraceae) in Tennessee, U.S.A. Flora, 236–237: 115–125. 10.1016/j.flora.2017.10.005