Morphological and allozyme variation of
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| Morphological and allozyme variation of Eidolon helvum (Mammalia: Megachiroptera) in the islands of the Gulf of Guinea
JAVIER JUSTE1,2∗, CARLOS IBA´ N˜ EZ2 AND ANNIE MACHORDOM3 1Departamento de Bioqu´ımica y Biolog´ıa Molecular IV, Facultad de Veterinaria, Universidad Complutense de Madrid, Madrid 28040, Spain; 2Estacio´n Biolo´gica de Don˜ana, CSIC, Apartado 1056, E-41080 Sevilla, Spain; 3Museo Nacional de Ciencias Naturales (CSIC), Jos´e Guti´errez Abascal 2, 28006 Madrid, Spain  Morphological and genetic variation is evaluated among populations of the bat, Eidolon helvum, in the islands of the Gulf of Guinea (Central Africa). The populations from the islands of Bioko, Pr´ıncipe, and Sa˜ o Tome´ do not show significant phenetic differentiation, although a trend towards a reduction of size is found in the latter two islands. The low genetic distances between populations, as well as their values of Wright’s fixation indexes, suggest that gene flow has hampered differentiation on these islands. In contrast, the population from Annobo´ n, the smallest and farthermost island, shows remarkable morphological and genetic differentiation. On the mainland, E. helvum displays unique migratory and dispersal behaviours, but migratory behaviour was not found in any of the island populations. The combination of selective forces in harsher oceanic environments and restricted gene flow among populations appears to have favoured the high degree of morphological differentiation of E. helvum on Annobo´ n. Due to the extended length of the dry season in Annobo´ n, an earlier achievement of sexual maturity—and consequently smaller size—may be advantageous in the absence of migration. The differentiation is more marked among females, which also suggests that selection may be linked to the reproductive pattern. The population of the island of Annobo´ n is herein described as a new subspecies, Eidolon helvum annobonensis subsp. nov.
2000 The Linnean Society of London ADDITIONAL KEY WORDS:—Genetic variation – island populations – taxonomy – Africa – fruit bats. CONTENTS
∗ Corresponding author: Javier Juste, at Seville address. E-mail: jjuste@compuserve.com 359 0024–4066/00/100359+20 $35.00/0 2000 The Linnean Society of London
INTRODUCTION The African straw-coloured bat, Eidolon helvum (Kerr, 1792), is the second largest fruit bat in Africa, with unique morphological (Andersen, 1912), ecological (Thomas, 1982), and reproductive (Bernard & Cumming, 1997) characteristics. A recent phylogenetic study suggests that Eidolon’s origin may be closer to the typically Asian Pteropus group than to any extant African fruit bat ( Juste et al., 1999). Like the Pteropus bats, E. helvum has narrow and pointed wings, a morphology particularly apt for long-distance flying (Rosevear, 1965). In fact, E. helvum is able to move in seasonal migrations as far as 2500 km in Sudan (Kock, 1969) or 1500 km in West Africa (Thomas, 1983). Possibly associated with its high vagility, Eidolon is a poorly diversified genus and only two species are currently recognized: the nominative E. helvum spread across the sub-Saharan continent (DeFrees & Wilson, 1988), and the endemic E. dupreanum from Madagascar (Bergmans, 1990). E. helvum is considered morphologically uniform throughout its vast distribution range. Only a population from Southern Arabia, E. h. sabaeum, is distinguished at the subspecific level (Bergmans, 1990), on the basis of its smaller size and heavier dentition (Andersen, 1912). Islands appear to be favoured by E. helvum along the African coast (Rosevear, 1965): it is the only fruit bat that has successfully colonized the four islands of the Gulf of Guinea in West Central Africa (Fig. 1) ( Juste & Iba´ n˜ ez, 1994). Its presence has been reported in all the islands (Bioko, Pr´ıncipe, Sa˜ o Tome´ and Annobo´ n), since the earliest scientific expeditions to this group (Barboza du Bocage, 1897, 1903; Cabrera, 1908). The maximum distance between the islands—and to the mainland—in the Gulf of Guinea is 350 km. This does not appear to present an insurmountable barrier to contact between the various island populations, and differentiation would not be expected. Nevertheless, Eisentraut (1964) found some cranial differences between specimens from Bioko and the mainland. Bergmans (1990) has suggested that the lack of migratory movements could have provided sufficient isolation to allow for the differentiation of this population. The status of E. helvum in the islands is still unknown because only a few scattered specimens from these populations exist in collections. Consequently, relationships among them, and the possible role that the migratory behaviour and high dispersal potential of E. helvum could play in defining their characteristics, are still to be assessed. 10°E 32 km
(2027 km2) 220 km Príncipe
146 km Río Muni
São Tomé (857 km2) Equator 280 km
180 km Annobón
(17 km2) 340 km
 Figure 1. Map of the Gulf of Guinea, Central West Africa.
The objectives of this work were: (1) to characterize morphologically and genetically the populations of E. helvum from all the islands; (2) to examine for patterns of variation within these two sources of variability, and for congruence between them; (3) to infer evolutionary relationships, as well as possible colonization patterns among these populations in the geographic context of the Gulf of Guinea. MATERIAL AND METHODS
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