Comparative anatomy and physiology of myrmecophytes: ecological and evolutionary perspectives

Figures & data

Table 1 Rewards provided by myrmecophytic plants

Extrafloral nectaries: secretory tissues generally located on vegetative tissues and are not involved in pollination. Nectar secreted is a mixture of compounds usually dominated by sugars and amino acids. Food bodies: cellular structures that serve as rewards for ants, and are rich in lipids and proteins. Domatia: plant structures that serve as nesting space for ants. All ant–myrmecophyte symbiotic associations have one thing in common, that is plants provide domatia, such as hollow thorns, petioles, stems, or modified leaves to their inhabitant ants.

Figure 1 Rewards produced by myrmecophytic plants to establish stable obligate symbiotic mutualisms with ants.

Notes: Extrafloral nectar of Acacia hindsii (A) and Acacia cornigera (B). Food bodies and a hollow thorn of Acacia sp. (C). A hollow stem of Macaranga bancana (D). Food bodies at the base of a leafstalk in Cecropia mexicana (E).

Table 2 Plant–ant obligate interactions in six plant groups

	Mesoamerican Acacia sp. (Leguminosae)	African Acacia sp. (Leguminosae)	Macaranga sp. (Euphorbiaceae)	Cecropia sp. (Moraceae)	Piper sp. (Piperaceae)	Leonardoxa africana (Leguminosae)
Mutualistic ant partner	Pseudomyrmex	Crematogaster	Crematogaster	Azteca	Pheidole	Petalomyrmex
Position of extrafloral nectar	Leaf base	Leaf base	Absent	Absent	Absent	Leaf base
Food bodies	Multicellular; rich in proteins	Multicellular	Multicellular; rich in lipids	Multicellular; rich in glycogen and lipids	Single cells; rich in lipids	Absent
Domatia	Hollow thorn	Hollow thorn	Hollow stem	Hollow stem	Hollow stem	Hollow internodes
Ant service	Defense	Defense	Defense	Defense, nutrient supply	Defense, nutrient supply	Defense

Notes: The American and African Acacia spp. are now assigned to the genera Vachellia, Senegalia, Mariosousa, and Acaciella.^133,134 Here, we will use the genus name Acacia to refer to American and African lineages to maintain consistency with previous studies.

Figure 2 Anatomical tissues typically recognized in extrafloral nectaries.

Abbreviations: E, epidermis; NP, nectary parenchyma; SNP, subnectary parenchyma.

Figure 3 Comparison of the anatomical structure of food bodies in Piper and Acacia cornigera.

Notes: In Piper (A), FBs are anatomically simple and involve the enlargement of single epidermal cells without any involvement of the underlying parenchyma. In Acacia cornigera (B), FBs are anatomically differentiated and show a specialized internal differentiation of cells. They are connected to the vascular tissue.
Abbreviations: EC, epidermal cell; E, epidermis; CC, cortex cell; FBs, food bodies; H, hypodermis, VS, vascular system.

Figure 4 Comparison of the anatomical structure between a hallow and an unhollowed stem.

Notes: The figure depicts differences in the anatomical structure between a hollow stem of a myrmecophytic plant (A) and an unhollowed stem of a myrmecophilic plant (B). Copyright © 2007 The Botanical Society of America. Adapted from Tepe EJ, Vincent MA, Watson LE. Stem diversity, cauline domatia, and the evolution of ant–plant associations in Piper sect. Macrostachys (Piperaceae). Am J Bot. 2007;94:1–11.⁹⁸
Abbreviations: C, crystal; VB, vascular bundle; WT, wound tissue.

Figure 5 Comparison of the anatomical structure of an ant domatia with that of a leaf lamina.

Notes: The figure depicts differences in the anatomical structure between an ant domatia (A) and a leaf lamina (B).
Abbreviations: PP, palisade parenchyma; E, epidermis; PT, parenchymatous tissue; PP, palisade parenchyma; SP, spongy parenchyma; VB, vascular bundle.

Table 3 Outstanding questions

– How do myrmecophytic plants mechanistically regulate the production of extrafloral nectar, food bodies, and domatia? – Do the rewards offered by myrmecophytic plants represent a specific response to the mutualistic ant partner? – Which signal, chemical or molecular, is used by myrmecophytes to identify the appropriate partner? – How are myrmecophytic plants able to sense the presence of non-mutualistic ants that enable them to cease the production of rewards? – How do plants and ants judge the quantity and/or quality of the resources provided by the partner, in order to express phenotypic plasticity in the provisioning of rewards and services that regulate an efficient reciprocal rewarding system?

Tepe EJ, Vincent MA, Watson LE. Stem diversity, cauline domatia, and the evolution of ant–plant associations in Piper sect. Macrostachys (Piperaceae). Am J Bot. 2007;94:1–11.

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