A Case for Selection for Delayed Fruit Maturation in Spondias (Anacardiaceae)

Field: Agricultural and Biological Sciences (Life Sciences)

Spondias radlkoferi J. Donn. Sm. of lower Central America and Panama is believed to have been derived from the more widespread S. mombin L. The fruits of S. mombin are both colorful and sweet, characteristic of the genus. S. radlkoferi, however, has fruits which remain green and are not sweet at maturity. Where the two species occur together on Barro Colorado Island, they are, nevertheless, equally abundant. Selection in S. radlkoferi appears to have favored longer periods of fruit maturation to extend the time of maturation into a period of the year marked by food scarcity. Apparently also because of the dearth of food and the resulting increase in the foraging activities by mammals there has been selection in favor of a fruit which is neither sweet nor colorful. THE GENUS Spondias (Anacardiaceae), with 8-12 species from tropical regions of the world, is renowned for its tasty, edible fruits (Adams 1972, Bailey 1949, Standley 1928). The fruits are customarily also colorful at maturity, usually yellow, orange, purple, or red. Recently, while conducting research toward a new Flora of Barro Colorado Island in the Panama Canal Zone, I encountered a species which is unusual in the genus. The species, S. radlkoferi J. Donn. Sm., was considered synonymous with S. mombin L. by Blackwell (1968) in the Flora of Panama. However, studies conducted on Barro Colorado Island during 1970 and 1971 and at the Missouri Botanical Garden since then have shown that the two species are distinct. Among the more important features distinguishing the two species are differences in flowering and fruiting time, fruit color, endocarp shape, sap color, bark texture, calyx pubescence, flower bud shape, number of styles, disk width, and type of pubescence, all summarized in table 1. Perhaps the most biologically significant feature separating the two species is the difference in flowering time. Initiation of flowering for both species varies somewhat from year to year, perhaps depending on climatic conditions. Spoidias momben begins losing its leaves in the earliest part of the dry season, usually from December to February. Before flowering is initiated, the species usually has a full complement of new but fully matured leaves. Flowering occurs principally between March and July. At the time flowering is initiated in S. radlkoferi its leaves are only partially developed so that flowers develop precociously or simultaneously with the new leaves. Spondias radlkoferi usually loses its leaves later than S. mombin 1 This study was supported in part by N.S.F. Grants GB34502 and GB-36202X. and begins to flower 4-6 weeks after S. mombin, principally April to July. While populations of both species are common on the island and have flowering periods lasting 4 months or more, individuals rarely flower more than 4-6 weeks. Despite the extended period of overlap in their flowering periods, there is no evidence that the two species hybridize. The characters separating them are quite uniform and consistent for two such close cogeners. Fruit development time for Spondias mombin averages about five months with the fruits maturing principally from July to October but especially during August and September. The drupe is obovoid to oblong, 2.5-3 cm long and orange at maturity with a thin exocarp and a fleshy, tasty, sweet mesocarp surrounding the tough, fibrous, apparently inedible seed. In contrast, Spondias radikoferi has fruits which require an average of six months to develop, maturing principally from September to December. At maturity the fruits are somewhat larger than S. mombin and of similar shape but do not turn orange. The mesocarp, though becoming moderately soft and fleshy, is not sweet or tasty like other Spondias species. Spondias radlkofeeri ranges from Guatemala to Panama. It is without question closely related to S. mombin which ranges throughout tropical America. Based on morphological data (Croat 1974), such as the reduction in number of styles, it appears likely to have been derived from S. mombin. It is also appropriate on this basis to presume that the species has lost the generically characteristic ability to develop colored, tasty fruits. This selection against such fruits can be explained by its phenological history. Phenological studies on Barro Colorado Island BIOTROPICA 6(2): 135-137 135 This content downloaded from 157.55.39.215 on Wed, 31 Aug 2016 04:26:57 UTC All use subject to http://about.jstor.org/terms TABLE 1. Characters separating Spondias mombin L. and S. radlkoferi J. Donn. Sm. S. mombin S. radlkoferi Pubescent parts of plant puberulent Pubescent parts of plant villous to (trichomes short and straight) velutinous Bark very coarse, deeply fissured Bark not deeply fissured, the strips the intervening corky periderm of periderm paper-thin, smooth hard and prominently raised Slash not producing cloudy, viscid Slash producing cloudy, viscid droplets droplets on inner bark within 30-60 seconds on inner bark Leaves generally fully developed at Leaves usually young at time of flowering time of flowering Leaves drying greenish Leaves drying blackish Leaf blades with a prominent Leaf blades lacking prominent submarginal submarginal collecting nerve collecting nerve Flowers with 4 or 5 (rarely 3) styles Flowers with 3 or 4 (rarely 5) styles Flower buds globular to obovoid Flower buds ovoid Calyx pubescent Calyx glabrous Pedicels puberulent Pedicels glabrous or with sparse long trichomes Disk ca 1 mm wide, narrower than Disk ca 2.3 mm wide, wider than stylar stylar clump clump Mature fruits orange, sweet and tasty Mature fruits green, edible but not very tasty and reported to be more acidic Old endocarps (persisting on ground Old endocarps oblong beneath tree for most of the year) obovoid (Croat 1969) show that the peak of fruiting for all species of trees is in April (fig. 1) and that, from this period until the end of the rainy season in December, there are fewer species of trees available with fruit. Smythe (1970), in food-trap sampling experiments, has shown that May is the month with the maximum number of species producing fruits on Barro Colorado Island. However, his data also indicate marked paucity of fruits throughout the remainder of the year, nearly all of which is the rainy season. The rainy season begins on Barro Colorado