The Somatic Chromosomes of Sophora fernandeziana (Fabaceae), an Endemic Tree from Robinson Crusoe Island

Abstract

The mitotic chromosome number and karyotype of Sophora fernandeziana (Phil.) Skottsb. (subfamily Papilionoideae) are reported for the ®rst time. The chromosome number, 2n ˆ 18, is the modal number reported for the genus. The chromosomes are small (average length 1.55G 0.23 mm) and bear no satellites. The intrachromosomal and interchromosomal asymmetry indices were A1 ˆ 0:26 and A2 ˆ 0:18, respectively. This symmetrical karyotype is composed of 7 metacentric‡ 2 submetacentric pairs. This species is related to S. tetraptera J. Mill. from New Zealand. Both share the same chromosome number; unfortunately comparative karyotype data are not available for S. tetraptera. Our data suggest that no changes in chromosome number have occurred during the speciation of S. fernandeziana, in accordance with previous studies of other endemic species in the Juan FernaÂndez ora and for island endemics in general. However, only a small percentage of actual karyotypes of island endemics have been studied, so generalizations about chromosomal evolution for such species are not yet well founded. Robinson Crusoe Island (Masatierra) is one of the three islands of the small Juan FernaÂndez Archipelago in Chile. It is located in the Paci®c Ocean, 667 km W of continental Chile at 33 S latitude, having an estimated age of ca. 4 million yr old (Stuessy et al. 1984). The archipelago is well known for the high level of endemism among vascular plants (ca. 63% of the ora [Marticorena et al. 1998]). Unfortunately, currently more than 62% of the ora is considered rare, and two species are already extinct (Stuessy et al. 1998). The ora is threatened by both anthropogenic and natural phenomena, including ®re, erosion, vegetation cutting, and continued introduction of animals and invasive plants (Stuessy et al. 1998). There are the only two endemic legume species that inhabit the archipelago. Both are trees in the genus Sophora L. and each of the main islands supports one species: S. masafuerana (Phil.) Skottsb. on Alexander Selkirk Island and S. fernandeziana (Phil.) Skottsb. on Robinson Crusoe Island. Sophora includes ca. 43 species (Polhill 1981, Sousa S. and Rudd 1993) partitioned among three sections. Sophora fernandeziana belongs to Sophora sect. Edwardsia (Salisb.) Taub., characterized by having a calyx without an upper lip, stamens and style exserted, lower petals all basically similar, standard not strongly re exed, and pods often four-winged. This section comprises about 10 species: one each on ReÂunion Island and Hawai`i, and the remainder on Lord Howe Island, New Zealand, and southwestern South America (Polhill 1981). Chromosomal surveys based on haploid chromosome numbers (Sanders et al. 1983, Spooner et al. 1987, Sun et al. 1990) have been made for 38 of the 156 native and endemic species of the Juan FernaÂndez Archipelago (Marticorena et al. 1998). However, 71 1 Support was provided from the National Science Foundation (U.S.A.), Consejo Nacional de Investigaciones Ciento ®cas y TeÂnicas (CONICET), and Consejo de Investigaciones Ciento ®cas y Tecnolo gicas de la provincia de Co rdoba (CONICOR) (Argentina). Manuscript accepted 31 March 2000. 2 Instituto Multidisciplinario de Biologo Âa Vegetal, C. C. 495, 5000 Co rdoba, Argentina. 3 The University of Connecticut, Department of Ecology and Evolutionary Biology, Storrs, Connecticut