Introduction
Rubiaceae
Asclepiadaceae
Nepenthaceae
Bromeliaceae
Orchidaceae
Polypodiaceae
Ecology and
evolution
Cultivation
References
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Rubiaceae
Highly specialized domatia are found in southeast Asian species of the family Rubiaceae. These relatives of coffee and gardenias are the classic ant-fed ant-house plants. Five genera -- Anthorrhiza, Hydnophytum, Myrmecodia, Myrmephytum, and Squamellaria -- are found from Thailand to Fiji, with the highest diversity in New Guinea (Huxley and Jebb, 1991). Myrmecodia and Hydnophytum species are the most commonly cultivated, although I saw an impressive Anthorrhiza echinella (syn. A. clemensii)specimen growing in the Botanical Garden at the University of Oxford in 1998.
The domatium of these epiphytes is the hypocotyl, the stem below the cotyledons, which becomes enlarged into a tuber. Chambers form within the tuber as tissue dies back in a process that is presumably determined genetically. The chambers have entrance holes which allow the ants access, and
in some cases, tiny pores for ventilation (Huxley, 1978; Huxley and Jebb, 1991).
Nutrients released by decay of the ants' waste, mostly leftover insect pieces, are
absorbed by special "warts" inside the chambers (Huxley, 1978; Rickson, 1979).
Myrmecodia species have separate smooth-walled chambers that
the ants inhabit and warty chambers that serve as debris dumps.
In Hydnophytum, the chambers are less specialized,
but the warts tend to be concentrated towards the end of
the chambers.
Although ants are the primary inhabitants
of Myrmecodia and Hydnophytum, many other organisms take advantage of the
domatia. Hydnophytum species with large chambers may be inhabited by frogs or
lizards (Huxley, 1978). In Australia, the Apollo Jewel
butterfly (Hypochrysops apollo apollo) is completely dependent
on Myrmecodia beccarii. The caterpillar of this species feeds exclusively
on the foliage and tuber of M. beccarii and lives within the tuber (Forster, 2000).
The ant inhabitants of the Myrmecodia tolerate the caterpillar, because they receive nutritious
excretions from its glands.
Two species of epiphytic shrubs have
a relationship with ant-house plants that parallels that of the Apollo Jewel Butterfly (Clausing, 1998).
Pachycentria glauca and Pachycentria constricta (Melastomataceae) both have seeds that are collected by ants and deposited near, or even within, the ant-plants. They also produce pearl bodies on their leaves and
stems that are eaten by ants. Pachycentria constricta may grow
lithophytically as well as epiphytically on Hydnophytum or Dischidia,
but at Bako National Park in Sarawak, Pachycentria glauca seems to grow
exclusively on Hydnophytum formicarum and Myrmecodia tuberosa (Janzen,
1974; Clausing, 1998). Janzen (1974) reported that P. glauca seeds often
germinate inside the chambers of Hydnophytum plants, and the Pachycentria
stems later emerge from an entrance hole. The roots of the Pachycentria
plants grow into the debris deposits and presumably compete with the ant-house
plants for the available nutrients. In these three-sided relationships, the
ants, Apollo Jewel butterfly, and Pachycentria plants all benefit at the expense
of the ant-house plants.
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