Solanum maternum Bohs

Solanum maternum belongs to a complex of closely related taxa from Bolivia, which includes S. roseum, S. unilobum, and S. betaceum, the cultivated tree tomato. Solanum maternum is morphologically very similar to S. betaceum, and the two species are difficult to distinguish from herbarium material alone. Differences include slightly longer and less dense pubescence, more shallowly cordate leaves, and slightly wider anther connectives in S. maternum. Living material differs from S. betaceum in flower and fruit color. Solanum maternum has purplish or pinkish corollas with a deep purple star at base, whereas S. betaceum generally has uniformly white or pinkish corollas. The fruits of S. maternum are yellow or light orange, not dark orange or reddish as is typical of S. betaceum, although some forms of S. betaceum may have yellowish or light orange fruits. Fruits of S. maternum usually have a prominent apiculum or “nipple” at the distal end; those of S. betaceum may be acute distally, but usually lack an obvious apiculum. The reddish or purplish corollas of S. maternum resemble those of S. roseum, but the latter species is nearly glabrous and has much larger seeds than S. maternum. Solanum maternum differs from S. unilobum in its corolla color (S. unilobum has greenish corollas), its unexpanded style and small stigma, and its more abundant pubescence.

Solanum maternum grows wild in upland areas of Bolivia in the Departments of La Paz, Cochabamba, and Santa Cruz. Solanum unilobum also occurs in this area, but is generally found at lower elevations (300-1700 m). Solanum roseum has been found only farther to the northwest in the Department of La Paz, and its range is probably not sympatric with that of S. maternum. Wild populations of S. betaceum have been located in southern Bolivia in Dept. Tarija and in adjacent areas of northwestern Argentina (L. Bohs, unpub. data). Thus far the ranges of S. maternum and S. betaceum do not appear to be sympatric, although more field work is needed to ascertain the extent of these species in southern Bolivia.

In addition to morphological similarity, crossing data indicate a close relationship among S. maternum, S. betaceum, S. roseum, and S. unilobum. Crossing studies show that S. betaceum will form fertile hybrids in at least some combinations with S. maternum, S. roseum and S. unilobum (Bohs, 1991; Bohs & Nelson, 1997). Solanum maternum, S. roseum, and S. unilobum are self-incompatible, whereas all accessions of S. betaceum, both wild and cultivated, that have been tested are self-compatible (Bohs, 1991; Bohs & Nelson, 1997; L. Bohs, unpub. data). Molecular sequence data also confirm that these four species are closely related; ITS sequences from the four taxa are identical or nearly so (Bohs, in press b).

The close morphological resemblance of S. maternum to S. betaceum suggests that S. maternum may be the closest wild relative of the tree tomato. Solanum maternum should be explored as a source of useful genes for improvement of the tree tomato, and may itself be valuable as an economic fruit crop.