Gesneriaceae:
Phytogeographical History
 

 

The geographical origin of Gesneriaceae is still enigmatic. One hypothesis is that the family is of Chinese (“Cathaysian”) origin. Though this may appear plausible at first sight because of the many (31) gesneriaceous genera endemic here and the up to three times higher number of genera in mainland Asia as compared to tropical SE Asia. However, this opinion does not take into account that (a) the generic concepts of many Chinese genera are very narrow, (b) that the centre of evolutionary diversification is uncritically equated with the centre of origin, and (c) that it is based solely on the consideration of Asiatic Gesneriaceae and ignores other parts of the family such as the New World Gesneriaceae. When considering these other parts it is almost inevitable to link the present distribution with continental drift and plate tectonics. Recently, Burtt (1998) has proposed the following "highly speculative" hypothesis. The essential points are:

  1. Gesneriaceae is a family of Gondwanaland origin.

  2. The Coronantheroid Gesneriaceae are a relict group. Its Australasian members have survived on the Australian plate.

  3. The Coronantheroid Gesneriaceae invaded the Americas via the Antarctic and southern South America and gave rise to the New World Gesneriaceae. While the Coronantheroid Gesneriaceae became nearly extinct (the three temperate South American genera being the last survivors), the Gesnerioid Gesneriaceae evolved explosively in the American tropics.

  4. The Australasian Coronantheroid Gesneriaceae gave rise to the present paleotropical Gesneriaceae. These moved northwards with the Indian plate and split very early in the Epithematoid and Didymocarpoid Gesneriaceae. Before this split, the vital mutation to anisocotyly must have been taken place.

  5. The presently small group of Epithematoid Gesneriaceae is a relict group that was once much larger and had a much wider distribution in Asia and Africa. Epithema tenue can be considered as the last remnant of Epithematoid Gesneriaceae on the African continent. Rhynchoglossum reached America (where it is now represented by R. azureum) via Africa from where it now has completely disappeared.

  6. On the way north, a part of Didymocarpoid Gesneriaceae spread to Madagascar and colonized mainland Africa from there.

  7. The Indian plate carried the Didymocarpoid Gesneriaceae finally to the Asiatic continent. Here a division took place between the plants of northern India and the south, probably as the result of desiccation. The northern group became established in the Sino-Himalayan area and spread from here, under active evolutionary diversification, east- and southeastwards to China and adjacent areas, as well as westwards to Europe. The southern group spread from S India and Sri Lanka into Sundaland and moved eastwards. Strong support for the west to east migration can be seen in the fact that no endemic genera (the state of the monotypic Sepikea is doubtful) are found east of Wallace’s line. Time was apparently too short for new genera to evolve in that area.

Though Burtt's hypothesis appears very plausible from a geographical point of view, it is hard to believe that the family is old enough to match the geological events of Gondwanaland breakup.

The next problem is the position of the Coronantheroid Gesneriaceae. Recent molecular studies (Smith 2005) suggest that this group is part of the New World Gesneriaceae, without any link to the Epithematoid and Didymocarpoid Gesneriaceae. It seems that the Coronantheroid Gesneriaceae have evolved in America and some have invaded secondarily the SW Pacific via the Antarctic region. They represent, so to speak, the long arm of New World Gesneriaceae reaching into the Old World. It is probably a “blind end”, having no phylogenetic connection with the “true” Old World Gesneriaceae.

Thirdly, regarding Epithema and Rhynchoglossum, it seems more plausible to explain the disjunct distribution of individual species by rather recent introduction events than by continental drift. The close relationship of Rhynchoglossum azureum with South Indian species (Mayer et al. 2003) suggests that the species has reached America rather recently, probably via transpacific trips or migrations of early Polynesians. This is also suggested by the current rare localities which are usually near former population centres of ancient dwellers along the Pacific coast.

Fourthly, the available molecular data do not suggest that the African Gesneriaceae have split off earlier than the Asiatic Gesneriaceae. In fact, the African Gesneriaceae seem to have evolved from an ancestral Eurasian group, from which the “Basal Asiatic-European genera” are the last remnants.

In conclusion, the geographical origin of the New and particularly Old World Gesneriaceae is still enigmatic. In some characters the New World Gesneriaceae appear to represent the more primitive and older group, but the separation of the Old World Gesneriaceae must have been taken place very early.

 

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