The gulls (tribe Larini) constitute a well-delimited, cosmopolitan group of birds including 50 extant species (following Sibley & Monroe’s, 1990, systematic treatment). They all share a similar general outlook, being medium-sized with nonspecialized bills, webbed feet, short legs and neck. They are all colonial, long-lived species, and some are amongst the most intensively studied birds (see for instance applied aspects in Blokpoel & Scharf, 1991; Isenmann et al., 1991 ; Meathrel et al., 1991 ; Spaans et al., 1991 ; Vermeer & Irons, 1991).
Gulls are widespread from tropical to arctic regions and, although all species are closely associated with wetlands or marine environment both during and out of the reproductive season, they have evolved under highly different ecological constraints and have adapted to many different habitats ( del Hoyo et al., 1996 ). This widely studied group thus shows enough diversity in several life-history or morphological traits for comparative analyses of these traits to be especially interesting ( Jouventin & Mougin, 1981).
A reliable phylogeny is a prerequisite for such analyses. In the past, three main studies have addressed the question of the relationships within the Larini, based, respectively, on plumage ( Dwight, 1925), behaviour ( Moynihan, 1959) and morphometrics ( Schnell, 1970a, b). Recently, Chu (1998 ) published a complete cladistic analysis based on 117 skeleton and 64 integument characters. Further hypotheses concerning the relationships within particular subsets of species have been put forward (see Cramp & Simmons, 1983, or del Hoyo et al., 1996 ). Despite this considerable interest, no general consensus exists on a phylogenetic arrangement of Larini. Most gulls are generally included in the genus Larus except several species believed to have diverged first (see del Hoyo et al., 1996 ). Within the genus Larus, two main groups have been long recognized (e.g. Dwight, 1925; Moynihan, 1959; Cramp & Simmons, 1983; Sibley & Monroe, 1990; del Hoyo et al., 1996 ). One includes large, white-headed species, the other small species having a dark hood or believed to have recently lost their hood. Several species have been variously assigned to either of these groups in previous treatments of gull relationships. This split between white-headed and black-headed species has recently been questioned by Chu (1998), who suggested instead a basal split between ‘larine’ gulls and ‘sternine’ gulls (this last group made of the species usually classified in genera other than Larus as well as some black-headed species). In Table 2 and Fig. 3, we present the taxonomic relationships between the species that we analysed according to Dwight (1925), Moynihan (1959) and Chu (1998). The phenetic analysis of Schnell (1970a, b) was excluded because this author clearly stated in his papers that his aim was not to reconstruct phylogenetic relationships among gulls.
The lack of consensus that is apparent when comparing the results of these studies and the uncertainty surrounding the relationships of several species leaves room for a molecular investigation of the phylogeny of the Larini based on DNA sequence data. Examples of such investigations, generally based on mitochondrial genes, are now numerous in birds (see Mindel, 1997, for reviews and examples). In the present study, we sequenced parts of the mitochondrial control region and of the cytochrome b gene. The cytochrome b gene is one of the most widely used for phylogenetic studies at the genus/family level. The mitochondrial control region has mainly been employed at the species level, but also once at the genus level ( Marshall & Baker, 1997). It is generally a fast evolving gene, with its ETAS domain and CSB domain being hypervariable regions whereas the central domain is less variable ( Baker & Marshall, 1997; Sbisàet al., 1997 ). In gulls, preliminary investigations showed that its level of differentiation, even between widely divergent species, was adequate for phylogenetic purposes. For the present work, we sequenced the central and CSB control region domains and a short segment from the cytochrome b gene.
According to DNA–DNA hybridization-based avian systematics ( Sibley & Ahlquist, 1990), the closest relatives of gulls within the Charadrii are the terns (Sterna and allied genera). We therefore sequenced specimens of two tern species and used published sequences from another Charadriiformes as outgroup representatives.
We analysed samples from 32 gull species, covering most species groups, to obtain a rather complete picture of species relationships within the Larini. Only the Ross’s gull (R. rosea) and Dolphin gull (L. scoresbii) were missing among the species with uncertain affinities, while no member of the L. crassirostris–L. belcheri–L. pacificus assemblage could be included.