Ecology
Rafflesia spp. occur at a range of elevations and are variably widespread across the Philippines from near sea level to high elevation. Most Rafflesia species in the Philippines are geographically isolated – often restricted to disjunct and inaccessible mountain forests – with limited opportunity for gene flow among populations (Pelser et al . 2019). Indeed, populations within species can be markedly genetically differentiated, such as R. panchoanaon Mount Labo compared with other populations on Luzon (Pelser et al. 2017). Meanwhile the recently delineated R. banaoana andR. leonardi occur at a different elevations, in habitats with distinct plant assemblages (Tobias et al . 2023). Unusually,R. balatociana co-occurs in the same broader area as three other taxa: R. banaoana , R. consueloae , and R. cf.aurantia in the Cordillera Central. This presents an opportunity for gene flow among sympatric species – something hitherto unreported in Rafflesia. The species in closest proximity is R. banaoana, which also occurs in high elevation moss forest in the southwest of Kalinga Province. While R. balatociana blooms predominantly in September to December, with a peak in October; R. banaoana blooms between March to June, with an apparent peak in May. Therefore, these species appear to co-occur in sympatry. Specimens of the two species flowering in synchrony outside these periods is possible. However, fruit-set is rare in Rafflesia species, which are all unisexual (with synchronous blooming of male and female flowers uncommon), and flower erratically, often in small numbers. Indeed, we have never observed fruits in R. balatociana or R. banaoana during several years’ field work and observation throughout the calendar year. Rafflesia consueloae seems to flower in synchrony with R. banaoana; data on the phenology of R. aurantia are absent. However, R. consueloae typically occurs at lower elevation (300-500 m) (Galindon et al . 2016), and R. aurantia occurs mainly in disturbed lowland dipterocarp forest, and lower mountain slopes (Barcelona et al . 2009a); neither has been observed in close proximity to R. balatociana .
Pelser et al. (2016) report the occurrence of distinctRafflesia taxa reportedly collected 5-10 m from one another, and the presence of R. lobata and R. speciosa in sympatry in Panay. The authors noted that these sympatric species often diverge significantly in flower size (as is the case for R. balatocianaand R. banaoana ), which could lead to pollinator incompatibility. In Malaysian Borneo, R. keithii and R. pricei co-occur in close proximity with no evidence of gene flow, as pointed out by Meijer (1984); meanwhile in Indonesia the authors have observed R. arnoldii, R. kemumu and R. bengkuluensis in the same district (albeit separately), again without any apparent intermediates; however the genetic relatedness of these taxa has not been investigated and they are morphologically similar. Hybridisation has not been reported in the genus Rafflesia; hybrids might be expected to be exceedingly rare, given the scarcity of most putative parent species, brief and erratic blooming, and the allopatric and sympatric barriers between them, as discussed above. A detailed molecular study is needed ascertain whether hybridisation has been, or is, an evolutionary driver in the genus unequivocally. In the case of R. balatociana, we consider hybridization with co-occurring species unlikely because of the lack of intermediate morphological features (stressed above, and see key), as well as ecology and phenology.
Little is known about the host specificity of Rafflesia . Data available from the Philippines indicate that the plants may be less host–specific than considered previously, and that host–parasite co–speciation may not have been a major driver of diversification in the genus (Pelser et al. 2016). The genus Tetrastigma is itself taxonomically complicated, which has further hindered our understanding of the host specificity of Rafflesia . In the Cordillera Central, our observations suggest that R. banaoanagrows on Tetrastigma cf. sepulchrei Merr. and T. loheri Gagnep; the Tetrastigma hosts of R. consueloae , and R. aurantia are unreported. We were unable to collect vouchers of the host of R. balatociana because there were no leaves within reaching distance; a common problem in the host identification with Rafflesia.