Tracing the colonization history of the Indian Ocean scops-owls (Strigiformes: Otus) with further insight into the spatio-temporal origin of the Malagasy avifauna

Background The island of Madagascar and surrounding volcanic and coralline islands are considered to form a biodiversity hotspot with large numbers of unique taxa. The origin of this endemic fauna can be explained by two different factors: vicariance or over-water-dispersal. Deciphering which factor explains the current distributional pattern of a given taxonomic group requires robust phylogenies as well as estimates of divergence times. The lineage of Indian Ocean scops-owls (Otus: Strigidae) includes six or seven species that are endemic to Madagascar and portions of the Comoros and Seychelles archipelagos; little is known about the species limits, biogeographic affinities and relationships to each other. In the present study, using DNA sequence data gathered from six loci, we examine the biogeographic history of the Indian Ocean scops-owls. We also compare the pattern and timing of colonization of the Indian Ocean islands by scops-owls with divergence times already proposed for other bird taxa. Results Our analyses revealed that Indian Ocean islands scops-owls do not form a monophyletic assemblage: the Seychelles Otus insularis is genetically closer to the South-East Asian endemic O. sunia than to species from the Comoros and Madagascar. The Pemba Scops-owls O. pembaensis, often considered closely related to, if not conspecific with O. rutilus of Madagascar, is instead closely related to the African mainland O. senegalensis. Relationships among the Indian Ocean taxa from the Comoros and Madagascar are unresolved, despite the analysis of over 4000 bp, suggesting a diversification burst after the initial colonization event. We also highlight one case of putative back-colonization to the Asian mainland from an island ancestor (O. sunia). Our divergence date estimates, using a Bayesian relaxed clock method, suggest that all these events occurred during the last 3.6 myr; albeit colonization of the Indian Ocean islands were not synchronous, O. pembaensis diverged from O. senegalensis about 1.7 mya while species from Madagascar and the Comoro diverged from their continental sister-group about 3.6 mya. We highlight that our estimates coincide with estimates of diversification from other bird lineages. Conclusion Our analyses revealed the occurrence of multiple synchronous colonization events of the Indian Ocean islands by scops-owls, at a time when faunistic exchanges involving Madagascar was common as a result of lowered sea-level that would have allowed the formation of stepping-stone islands. Patterns of diversification that emerged from the scops-owls data are: 1) a star-like pattern concerning the order of colonization of the Indian Ocean islands and 2) the high genetic distinctiveness among all Indian Ocean taxa, reinforcing their recognition as distinct species.


Background
The island of Madagascar is considered a biodiversity hotspot with an intriguing endemic fauna [1]. The origin of the island's peculiar and highly unique fauna can be explained by two contrasting processes: vicariance when Madagascar became separated from the African landmass approximately 165 mya and from India approximately 88 mya [2,3], or over-water dispersal from the African, Australian and Eurasian landmasses. In near proximity to Madagascar are several archipelagos, which have different geological histories. The eastern islands in the Seychelles archipelago are of granitic origin and likely broke off from India and reached their current position some 55-75 mya when India drifted northwards [4], while the western portion of the Seychelles is comprised of recent atolls. In contrast, the volcanic Comoros archipelago is of relatively recent age (0-11 mya), hence the only plausible explanation for the colonization of its biota is by over-ocean dispersal from Africa, Australia, Madagascar or Eurasia.
The origin, timing and modes (vicariance versus overwater dispersal) of colonization by animals of Madagascar and in some cases the surrounding Comoro islands have been the focus of several research programs [5][6][7][8][9][10]. Given the antiquity of the break-up of Indo-Madagascar from Africa, which considerably pre-dates the first known fossil records of most modern families or genera of animals, in particular vertebrates, it is now assumed that the majority of extant groups arrived on Madagascar via over-water dispersal [11]. From recent phylogenetic studies, as well as traditional taxonomy, it is inferred that most of the Malagasy avifauna originated from African ancestors, although colonization events from Eurasia and Australia have also been documented for flying organisms such as bats and birds [12][13][14][15][16].
Interpreting the origin of certain components of the Malagasy avifauna is in many cases difficult when based on current taxonomical classifications alone as several genera of birds are shared between Madagascar, Africa and Eurasia. Among these are scops-owls of the genus Otus. As currently defined, Otus is present in five biogeographic areas (Indo-Malaya, Afrotropics, Nearctic, Neotropics, Palearctic) [17]. However, monophyly of Otus (sensu [17]) is uncertain; some molecular studies suggested that the New World Otus (Megascops-including Otus flammeolus, see [18]), which differ from Old World Otus by song type, are genetically more closely related to the widespread owl genera Strix and Bubo [19,20] whereas the African Whitefaced Owl (O. leucotis) has closer affinities with the genus Asio [20]. The above-mentioned taxa excluded, Otus species have their center of diversity in Eurasia (26 species); secondary radiations occur in the Indian Ocean (six or seven species) and Africa (four species). To our knowledge, the earliest known Otus fossil, a distal end of the right humerus, is from western Kenya and dates from the Miocene (16.5-18.5 mya; [21]). This partial fossil is morphologically close to O. senegalensis, but its relationship among members of the genus is not clear. The western Indian Ocean taxa are often thought to constitute a superspecies (rutilus group) of five or six species [22] [23] for a discussion about phylogeography and taxonomic status of the two Malagasy forms), and O. insularis from the granitic Seychelles. The latter species shows apparent affinities, based on vocalization data, to the Indonesian O. magicus [24]. Most [25][26][27][28], while O. moheliensis was first described in 1998 [29].
The evolutionary and biogeographic history of the Indian Ocean Otus taxa has not been the focus of a phylogenetic study. In this paper, we use two nuclear introns (myoglobin intron-2 and TGFB2 intron-5) and four mitochondrial protein coding-genes (ND2, ND3, ATP6, cytochrome-b) in order to propose a first multi-locus phylogeny of scops-owls and to track their colonization history of the western Indian Ocean islands in space and time. We additionally compare the biogeographic affinities of Malagasy scops-owl species to those of other Malagasy avian lineages which have been the focus of recent genetic studies [12,13,16]. The topologies obtained from the nuclear loci were very similar to each other, delineating the primary clades without achieving resolution at the tips (see Additional Files 1 and 2). The 50% majority consensus rule tree obtained from the Bayesian analyses ( Figure 1, -ln = 4638.29) and Maximum Parsimony strict consensus tree (105675 equally parsimonious trees of 412 steps, CI = 0.82, RI = 0.86) of the two concatenated nuclear loci provided a well-resolved topology for inter-generic relationships as well as some resolution of relationships among the primary Otus lineages but failed to provide resolution among members of the Indian Ocean radiation.

Sequence properties
The concatenated mitochondrial sequences retained for analyses were 2983 bp long (1047 bp for ND2, 684 bp for ATP6, 351 bp for ND3 and 901 bp for cytochrome-b) and correspond to the positions 5246 to 6281 (ND2), 9240 to 9923 (ATP6), 10776 to 11120 (ND3), and 15011 to 15911 (cytochrome-b) of the Gallus gallus mitochondrial genome sequence [30]; GenBank accession number X52392). The ATP6 and cytochrome-b sequences contained no insertions, deletions and stop-codons in the reading frame. The ND2 sequence of Aegolius acadicus exhibits a two-codon insertion (CAA ACC) just before the stop codon. All the ND3 sequences exhibited the pyrimidine insertion (T for O. capnodes, C for all other species analyzed) previously reported for several clades of birds [31]; this extra-nucleotide was removed before phylogenetic analyses. Partitioning the gene by codon positions significantly improved the fit of models to the data for all four mitochondrial loci, as inferred from the Bayes Factor

Phylogenetic results
The individual trees obtained from the two nuclear introns and mitochondrial data sets were very similar to each other and no incongruence was detected between the nuclear and mitochondrial topologies (Figure 1), according to the criteria defined in the Material and Methods section. Further, there was usually strong congruence for nodal support among the different methods. Most of the nodes present in the 50% majority-rule consensus tree resulting from the Bayesian analyses performed on the concatenated data set (partitioned by gene and codon position-14 partitions-, -ln = 32195.91, Figure 2) were very well supported in the parsimony analyses too (two equally most parsimonious trees, 6340 steps, CI = 0. 44    we regard the discrepancy between the mitochondrial and nuclear trees as being due to the different effective popu-lation size of the markers (the mitochondrial genome has an effective population size that is one fourth of the nuclear genome) and random processes (coalescence). The basal split within Otus sensu stricto occurred about 11.7 mya (95% HPD = 6.0-19.0) and the divergence between the two primary clades about 9.3 mya (95% HPD = 4.7-15.1, Figure 2).

Phylogeny of Otus sensu stricto
Previous studies [18,20] highlighted that several species usually assigned to Otus (Ptilopsis leucotis, Megascops) are not directly related to Otus. Our analyses, using independent samples and additional genes, confirmed the lack of direct relationships among these three lineages. Given that these results were previously suggested, we do not discuss them further.
Our analyses based on nuclear and mitochondrial sequence data provide the first phylogenetic hypothesis on the diversification of Otus sensu stricto. We suggest that Otus consists of at least three primary lineages.

Phylogeny and origin of the western Indian Ocean Otus
Data accumulated in recent years on the biology and distribution of the western Indian Ocean scops-owls [26][27][28][29] together with our analyses based on DNA sequences of six  [25,28,29]. In addition we find good support for Asian biogeographic affinities of the western Indian Ocean islands Otus spp., and highlight that they may constitute a paraphyletic assemblage.
The Indo-Malayan O. sunia formed a well-supported clade with O. insularis from the granitic Seychelles, which was very closely related to the Indian Ocean lineage, the latter forming a sequentially paraphyletic assemblage. The nested position of O. sunia/O. insularis within the Malagasy-Comorian clade, supported by a posterior probability of 0.98 (but not by MP bootstrap percentage), suggests a recent re-colonization of the mainland from an islanddistributed ancestor. Yet, we also acknowledge that posterior probabilities could be misleading when short internodes/polytomies are involved [33], which is the case here, and we await the implementations of reversiblejump Markov chain Monte Carlo algorithms to explore these aspects [33].
It is also worth noting that in a similar biogeographic comparison of a different group of birds, non-monophyly of western Indian Ocean taxa was also retrieved by Warren et al. [13]; the Indo-Malayan bulbul Hypsipetes madagascariensis was nested within the western Indian Ocean taxa, suggesting a similar scenario to the one we present for Otus. Colonization of the mainland from an island distributed ancestor is generally regarded as unlikely because: 1) mainland taxa are considered more competitive than island taxa and, 2) insular populations are smaller and produce fewer emigrants compared to those on continents [34,35]. However, empirical cases of continental re-colonization from islands are accumulating [16,[36][37][38], including cases from Madagascar [39]; this suggests that islands can also act as colonization sources for continental faunas. Data on the phylogeographic structure within the Indo-Malaya O. sunia complex may help to decipher which hypothesis (multiple colonizations of the Indian Ocean islands or re-colonization of the mainland) best explains the current pattern. Indeed, if strong and ancient phylogeographic structure occurs within O. sunia, we would expect two independent colonizations of the western Indian Ocean islands. In contrast, if genetic data indicate weak differentiation among O. sunia populations and patterns of population expansion from western to eastern Indo-Malaya, this would support the hypothesis of re-colonization of the mainland from the Seychelles Islands.

Vocalisations as a tool to infer evolutionary relationships among scops-owls
The genus Otus is fairly homogeneous in plumage relative to many bird genera. However, the Indian Ocean taxa show strong differences from each other in structural, plumage and vocal characters [26,28]. Considering, for example, only the songs of the Comorian taxa (which inhabit islands as close as 50 km from each other), O. pauliani gives a very long series of chaw notes repeated at about 2/sec; O. moheliensis a sequence of hisses; O. capnodes a high-pitched whistled peeooee; and O. mayottensis a series usually of 3-11 deep, single hoots. Intuitively, these differences may be used to argue against close evolutionary relationships; however, our data indicate that the relationship is indeed close in all cases, as could be predicted from the islands' proximity to one another. This study confirms that the vocal and morphological differences are indeed associated with distinct evolutionary lineages, but suggests that they are not related in any simple or obvious way to the evolutionary distance between these lineages, and therefore must be used with caution in identifying affinities between taxa (or lineages). Therefore, we highlight here that the close relationship suggested by Marshall [24] between O. insularis and O. magicus should be further tested using molecular data, especially if we consider the considerable distance between the two areas (over 6000 km).
Our estimate (3.6 mya, 95% HPD: 1.8-6.0) for the timing of colonization of the Indian Ocean islands by Otus coincides with estimates of at least seven other lineages of birds ( Table 2), suggesting that the Indian Ocean islands avifauna was highly enriched at that time. The period associated with these multiple independent colonizations corresponds with the emergence of the volcanic islands of the Comoros archipelago (Mohéli 5 mya, Anjouan 11.5-3.9 mya, Mayotte 11.5-7.7 mya and Grande Comore 0.5 mya; [50,51], thus providing possible stepping stones for dispersal between Africa and Madagascar. This possibility is also highlighted by the fact that several species or populations with African biogeographic affinities (Streptopelia capicola, Turtur tympanistria, Turdus bewsheri) [53][54][55], colonized the Comoros islands but the colonization of Madagascar has not yet been achieved.
All faunal exchanges that unambiguously involve Madagascar and Indo-Malaya, or Madagascar and the Seychelles occurred during the last 3.5 mya [ [13,49] Subfossil remains of three extinct small owl species have been described from the Mascarene Islands (La Réunion, Mauritius, Rodrigues). Based on certain osteological features, these three species have been included in their own genus, Mascarenotus, which has been suggested to be derived from Otus [59]. The relationships of Mascarenotus with respect to the other owl lineages still needs clarification, as this genus could represent a recent and derived off-shot of the western Indian Ocean lineage, possibly a first off-shot of the colonization process from the Sey- Anas Australasia No divergence date [50] mya: million years ago chelles to more westerly regional islands or even an unrelated lineage of owls.
Whereas the continental biogeographic affinities of certain western Indian Ocean island bird taxa seem largely resolved, the timing of colonizations or faunal affinities among volcanic islands are less well understood and the only aspect that is emerging is the absence of a common pattern, whatever the initial geographic origin. Indeed, all the studies that have been conducted so far indicate explosive diversification and a lack of resolution among the Comoros islands species. These data indicate that once the initial colonization was successful on any of the islands in this archipelago, dispersion and then diversification between nearby islands occurred quickly and randomly. The lack of biogeographic structure at the archipelago scale may thus be partly explained by the geographical arrangement of islands and the short inter-islands distances (maximum distance between two islands in the Comoros archipelago is 90 km) that probably favored colonization by alternative routes.
One final factor that could hide common patterns of diversification is unequal rates of extinction and recolonization across lineages. Indeed, not all lineages would face the same risk of extinction on islands as their characteristics (ecological requirements, population size) often considerably differ. For example, middle-sized birds, like owls (70-120 g), may be more prone to extinction on islands than small birds, like sunbirds (12-15 g) [60]. Yet, even if uneven extinctions rates occurred amongst these lineages, it can be concluded that the overall diversification pattern in the Indian Ocean islands is star-like.

Conclusion
Our analyses revealed the occurrence of multiple synchronous colonization events of the Indian Ocean islands by scops-owls, at a time when faunistic exchanges involving Madagascar was common as a result of lowered sea-level that would have allowed the formation of stepping-stone islands. Patterns of diversification that emerged from the scops-owls data are: 1) a star-like pattern concerning the order of colonization of the Indian Ocean islands and 2) the high genetic distinctiveness among all Indian Ocean taxa, reinforcing their recognition as distinct species.

Taxonomic sampling
We obtained tissue samples from all western Indian Ocean Otus taxa, as well as samples from several Indo-Malayan and Afrotropical species (Table 3), focusing on as many super-species complexes as possible (sensu [22]). We obtained tissues for seven of these super-species complexes. We were unable to obtain samples of the distinctive O. rufescens and O. sagittatus, as well as representatives of four super-species with localised and distant distributions relative to the western Indian Ocean (brooki/angelinae from Sumatra/Java/Borneo; mantananensis/magicus from the Lesser Sundas/Philippines/Mollucas; collari/ manadensis/beccarii from Sangihe/Sulawesi/Biak and enganensis/alius/umbra from islands off Sumatra). Since the monograph of Marks et al. [22], one further Otus species, O. thilohoffmani has been described from Sri Lanka [61]. This species is only known in museum collections by the type specimen, deposited in the National Museum Colombo (Sri Lanka), and based on morphology, has been suggested to be related to either O. rufescens or O. spilocephalus [61]. As a consequence, we did not have access to a tissue sample of this newly described species. With the exception of O. magicus, considered by some authors to include O. insularis because of similarities in vocalizations [24], none of the species we were unable to sample have been considered closely related to the Indian Ocean taxa [22]. We included two individuals per color morph for O. rutilus (sensu [23]). Representatives of the Strigidae genera Aegolius, Athene and Glaucidium (Surniinae), Asio (Asioninae), and Bubo, Strix, New World 'Otus' (Megascops) and the African White-faced Owl ('Otus' Ptilopsis leucotis) (Striginae) were included as proximate outgroups. We rooted our trees using sequences from a representative of the Tytonidae (Barn Owl Tyto alba), which has been recovered as the sister-group of the Strigidae in molecular and morphological analyses [62,63].

Phylogenetic analyses
Molecular phylogenies were estimated using parsimony (P) and model-based approaches (maximum likelihood [ML], and Bayesian inferences [BI]), as implemented in PHYML v2.4 [71] and MRBAYES 3.1 [72][73][74]. Parsimony analyses were conducted with PAUP v4.0b10 [75] using the heuristic tree bisection and reconnection branchswapping (TBR) algorithm with 100 random addition replicates. Likelihood models were estimated with MRMODELTEST 2.0 [76] using the Akaike Information Criterion [77]. The selected models are listed in Table 4. Clade support in the individual gene trees for the ML and MP analyses was assessed by 1000 non-parametric bootstrap replicates [78]. The six gene regions sequenced differ considerably in their properties and substitution dynamics, as inferred from the parameters of the models (Table  4). Consequently, analyses of concatenated data set were only performed using a mixed-model strategy. The relevance of partitioning the data set by gene and/or codon position was assessed with the Bayes Factor (B F ) [79,80]. Fourteen partitions (myoglobin intron-2, TGFB2 intron-5, first, second and third codon position of each of the four mitochondrial genes) were considered for the Bayesian concatenated analyses according to the functional properties of the markers. Bayesian analyses for the concatenated data set were performed allowing base frequencies, rate matrix, shape parameter and proportion of invariable sites to vary between the partitions (using the unlink and prset commands). Between four and six incrementally heated Metropolis-coupled MCMC chains were run for 15 million generations with trees sampled every 100 generations. The first 2*10 6 generations (20000 trees) were discarded ('burn-in' period) and the posterior probabilities were estimated from the remaining sampled generations. The default temperature for chain heating (T = 0.2) resulted in not satisfactorily mixing among chains for the concatenated data set; we therefore lowered the temperature to T = 0.05, which resulted in swap frequencies between chains within the 20-70% interval. Two independent Bayesian runs initiated from random starting trees were performed for each data set, and the log-likelihood values and posterior probabilities were checked to ascertain that the chains had reached convergence. We also checked that the Potential Scale Reduction Factor (PSRF) approached 1.0 for all parameters and that the average standard deviation of split frequencies converged towards zero. We detected significant incongruence between the individual gene trees by comparing the topologies and nodal support obtained under different analytical methods (ML, BI). Criteria for incongruence were set at 70% for the bootstrap values [81], and at 0.95 for posterior probabilities [72].

Molecular dating analyses
Owls have a very rich fossil record [22,82], yet, the taxonomic history of most of these taxa remains controversial and, to our knowledge, no cladistic analyses including fossil and modern taxa has been conducted to date. This hampers the use of most of these fossils as calibration points in our analyses. The family Strigidae is usually divided into three subfamilies or tribes: Striginae, Surniinae and Asioninae [21]. These three subfamilies are defined by a combination of shared characters [ [82] fide [83]]. The least inclusive of these three subfamilies, the Asioninae, consists of three genera and nine species (two of the genera, Pseudoscops and Nesasio are endemic to Jamaica and the Solomon Islands, respectively, whereas Asio is widely distributed). The Asionae differ osteologi-  cally from other owls by having: 1) the anterior rim of the internal trochlea not protruding more anteriorly than the anterior rim of the external trochlea, 2) the external calcaneal ridge bent posteriorly, 3) bony loop broad and 4) tubercle for Musculus tibialis antiquus displaced externally [83]. The most ancient fossil having this combination of characters Intulula tinnipara, has been dated from the Early Miocene (23.7-16.4 mya) [84]. We therefore used this date as a minimum age for the split between Asio otus (the member of the Asioninae we sampled) and its closest relative. As a second calibration point, we used the split between O. pauliani (endemic to Grande Comore, a volcanic island) and its closest relative at 0.5 mya. This date corresponds to the emergence of Grande Comore [51], and is thus the oldest possible age for the colonization of the O. pauliani lineage. We used these two calibration points in combination.
We used BEAST V1.4.6 [85][86][87] to estimate the divergence dates within the genus Otus. We assigned the best fitting model, as estimated by MRMODELTEST2, to each of the six loci. We used an exponential distribution for the fossil calibration bound [88]. We set the lower bound of the exponential distribution to 16.4 mya, which correspond to the lowest bound of the Early Miocene epoch and the expo-nential mean to 2.5, so that the 95% distribution probability fell within the 23.7-16.4 mya interval, corresponding to the Early Miocene, which is the epoch for the first Asioninae fossil. For the geological calibration point (emergence of Grande Comore), we used a normal distribution with the mean and standard deviation set to 0.5 mya and 0.1 mya, respectively. We assumed a Yule Speciation Process for the tree prior and an Uncorrelated Lognormal distribution for the molecular clock model [86]. We used default prior distributions for all other parameters and ran MCMC chains for 75 million generations, as the effective sample size for some parameter estimates was not large enough using the default length (10 million generations).

Authors' contributions
JF did part of the laboratory work, edited and aligned the sequences, performed the phylogenetic and dating analyses and wrote the first draft of the manuscript. JMP did part of the laboratory work and helped to draft the manuscript. SMG provided tissue samples for the study, wrote several sections of the manuscript and commented on multiple versions. VB collected tissue samples in the field and commented on the manuscript. MM assisted with sample collection and with the draft of the manuscript.