Leonardoxa, AFRICANA

Mckey, Doyle B., 2000, Leonardoxa africana (Leguminosae: Caesalpinioideae): a complex of mostly allopatric subspecies, Adansonia (3) 22 (1), pp. 71-109 : 72-73

THE LEONARDOXA AFRICANA COMPLEX

The range of Leonardoxa africana extends from Gabon and Equatorial Guinea northward through southwestern Cameroon and into extreme southeastern Nigeria. The species has long attracted attention as a myrmecophyte ( BEQUAERT 1922; MILDBRAED 1922; SCHNELL & BEAUFORT 1966; ELIAS 1980; MCKEY 1984; LÉONARD 1993). SCHNELL & BEAUFORT (1966), however, noted that swollen-stem myrmecodomatia were present only in some of the specimens from Cameroon they examined and were consistently absent from specimens collected from elsewhere in the range of the species, notably Gabon. LETOUZEY, the botanist who collected most widely in Cameroon, also noted the geographic heterogeneity of L. africana in presence vs. absence of swollen internodes ( LETOUZEY 1985; see also notes accompanying numerous specimens and unpublished field notes).

Field, greenhouse and herbarium studies reported on here demonstrate that L. africana is heterogeneous in this and many other characters. The variation concerns several traits that are obviously related to symbiosis with ants (ant-domatia, foliar nectaries), as well as other vegetative char- acters not directly related to ant-plant interactions (architectural differences related to rhythms of growth), and some floral characters (cauliflory, flower colour).

Some of the characters examined here merit special attention. Traits of myrmecodomatia have not often been used in taxonomic studies of groups including myrmecophytes, possibly because botanists have often mistakenly regarded these structures as galls, induced or altered in form by insects or other agents (see, for example, SCHNELL & BEAUFORT 1966).

In Leonardoxa , there is variation among the taxa recognised here not only in presence or absence of ant-domatia, but also in several aspects of their structure, and in the timing of onset of expression of these specialised stem swellings in the ontogeny of the plant.

Traits related to the phenology of vegetative growth are likewise rarely considered in taxonomic studies. Rhythmic growth of shoots characterises all L. africana , but the taxa recognised here differ in the number of internodes produced in each spurt of growth (a difference with architectural consequences, evident on both living plants and herbarium specimens) and in the degree to which growth is synchronised among branches (evident only on living trees). Differences in all these traits persist when plants are grown in greenhouses without ants, in common gardens, and in the one known instance when two taxa occur together in the same habitat. Differences in these traits are genetically based and taxonomically informative.

Discontinuities in variation in these and other characters consistently reveal a pattern of four distinct taxa, mostly allopatric in distribution. These new taxa are here described as subspecies of L. africana , reflecting the opinion that these taxa are not yet completely differentiated and reproductively isolated species. Three of the four subspecies are myrmecophytes, with swollen stems that form myrmecodomatia and house symbiotic ants. The fourth is not a myrmecophyte but, like the other subspecies (as well as many other Detarieae ), possesses foliar nectaries, which attract a diversity of non-resident ants. Experimental and observational field studies ( MCKEY 1991; GAUME et al. 1997, 1998; GAUME & MCKEY 1998, 1999; GAUME 1998) show that the four taxa recognised here are not only morphologically but also ecologically distinct. Although apparently only incompletely differentiated, they appear to be following independent evolutionary trajectories. This is most clearly illustrated by the coexistence in at least one site of the nonmyrmecophyte and the most highly specialised of the myrmecophytes (degree of specialisation being defined in terms of the number of new characters derived since divergence from the common ancestor). In sympatry, these two taxa remain morphologically and ecologically distinct, suggesting the existence of barriers to gene exchange. Molecular genetic studies (BROUAT et al., in press; C. BROUAT, unpublished data) confirm the distinctness of these coexisting populations and support the taxonomic separations made here on morphological grounds. These studies also suggest past or present gene flow between these taxa.