Background: The transition from outcrossing to selfing has long been portrayed as an ‘evolutionary dead end’ because, first, reversals are unlikely and, second, selfing lineages suffer from higher rates of extinction owing to a reduced potential for adaptation and the accumulation of deleterious mutations. We tested these two predictions in a clade of Madagascan Bulbophyllum orchids (30 spp.), including eight species where auto-pollinating morphs (i.e., selfers, without a ‘rostellum’) co-exist with their pollinator-dependent conspecifics (i.e., outcrossers, possessing a rostellum). Specifically, we addressed this issue on the basis of a time-calibrated phylogeny by means of ancestral character reconstructions and within the state-dependent evolution framework of BiSSE (Binary State Speciation and Extinction), which allowed jointly estimating rates of transition, speciation, and extinction between outcrossing and selfing. Results: The eight species capable of selfing occurred in scattered positions across the phylogeny, with two likely originating in the Pliocene (ca. 4.4–3.1 Ma), one in the Early Pleistocene (ca. 2.4 Ma), and five since the mid-Pleistocene (ca. = 1.3 Ma). We infer that this scattered phylogenetic distribution of selfing is best described by models including up to eight independent outcrossing-to-selfing transitions and very low rates of speciation (and either moderate or zero rates of extinction) associated with selfing. Conclusions: The frequent and irreversible outcrossing-to-selfing transitions in Madagascan Bulbophyllum are clearly congruent with the first prediction of the dead end hypothesis. The inability of our study to conclusively reject or support the likewise predicted higher extinction rate in selfing lineages might be explained by a combination of methodological limitations (low statistical power of our BiSSE approach to reliably estimate extinction in small-sized trees) and evolutionary processes (insufficient time elapsed for selfers to go extinct). We suggest that, in these tropical orchids, a simple genetic basis of selfing (via loss of the ‘rostellum’) is needed to explain the strikingly recurrent transitions to selfing, perhaps reflecting rapid response to parallel and novel selective environments over Late Quaternary (= 1.3 Ma) time scales.

Majority-rule consensus phylogenetic tree Bulbophyllum clade C (Figure 2)Majority-rule consensus of Madagascan Bulbophyllum clade C (sects. Calamaria, Humblotiorchis, Bifalcula) estimated under Bayesian Inference (MRBAYES version 3.1.2)Majority-rule consensus tree Bulbophyllum clade C (Figure 2).nex5,000 post-burn-in trees of the BI analysis (Figure 3A)Last 5,000 post-burn-in trees of the BI analysis used for ancestral character reconstruction (in BAYESTRAITS version 1.0)5,000 post-burn-in trees of the BI analysis.nex1,000 dated BEAST trees (Figure 3,4; Table 3,4)1,000 secondarily calibrated BEAST (version 1.6.1 ) trees resampled at a lower frequency (LOGCOMBINER version 1.6.1). Used for ancestral character reconstruction (Figure 3B), LTT plot (Figure 3), diversifcation analyses (Figure 4,Tab.3,Tab.4) using R packages APE and DIVERSITREE.1,000 dated BEAST trees.nexChronogram of Madagascan Bulbophyllum clade C (Figure 3C)Secondarily calibrated BEAST version 1.6.1 chronogram of Madagascan Bulbophyllum clade C.Chronogram of Madagascan Bulbophyllum clade C.nex