Isopora, Studer, 1878

Santodomingo, Nadiezhda, Wallace, Carden C. & Johnson, Kenneth G., 2015, Fossils reveal a high diversity of the staghorn coral genera Acropora and Isopora (Scleractinia: Acroporidae) in the Neogene of Indonesia, Zoological Journal of the Linnean Society 175 (4), pp. 677-763 : 695-696

AND ISOPORA

Molecular phylogenetic reconstructions have consistently shown that Isopora is a sister clade of Acropora ( Wallace et al., 2007; Fukami et al., 2008; Kitahara et al., 2010). The first appearance of Acropora in the fossil record is at 66 Ma ( Carbone et al., 1993), implying that the divergence of the two clades must have occurred during the Mesozoic. However, the earliest occurrence of Isopora is at 17.9 Ma during the Early Miocene. This long gap of more than 50 Myr in an apparently highly preservable taxon might be a result of misidentification of Isopora fossils as Acropora , as was the case for two Caribbean species ( Budd & Wallace, 2008). Another possibility is that Isopora was much rarer than Acropora , so less likely to be preserved and discovered in the fossil record. Alternatively, future inclusion of additional genes in analysis of modern material might result in reassessment of Isopora as paraphyletic with respect to Acropora . The presence of a single axial corallite was long thought to be diagnostic for Acropora (although present in a few species of Isopora, Wallace et al., 2007 ; Budd & Wallace, 2008), but axial corallites have recently been discovered in one species of the genus Astreopora , Astreopora acroporina Wallace, Turak, & DeVantier, 2011 . Astreopora is usually placed in a basal position within the family Acroporidae according to molecular phylogenetic reconstructions ( Fukami et al., 2000, 2008; Le Goff-Vitry, Rogers & Baglow, 2004; Kitahara et al., 2010). The presence of a single axial corallite in the basal genus Astreopora when combined with other characteristics typical of terminal genera in the family have been used to suggest that the genes controlling the expression of this character appeared earlier in the evolution of acroporids than previously thought. An alternative hypothesis is convergent evolution driven by environmental factors in small and isolated populations ( Wallace et al., 2011), as axial corallites have also been described in the unrelated merulinidae species Cyphastrea decadia Moll & Borel Best, 1984 , but further investigation is required to provide evidence to reject either hypothesis. So, it remains a challenge for palaeontologists to fill in the Paleogene gap of the fossil record of Isopora , as well as to further study the fossil record of the extinct genus Dendracis and the recently incorporated genus Alveopora , which was formally placed in the family Poritidae ( Fukami et al., 2008; Kitahara et al., 2010). A comprehensive study of the fossil record of Acroporidae is needed to better understand the ancestral states of basal groups of this family and give more insights into the hypothesis of evolution of the axial corallite and its implications for the success of the genus Acropora in modern reefs.

Molecular phylogenetic studies have also shown that species within some of the morphological species groups as defined by Wallace (1999) do not conform to monophyletic groups, and even more dramatically, many Acropora species are paraphyletic ( van Oppen et al., 2001; Richards et al., 2008, 2013; Chen et al., 2009). Hypotheses to explain the incongruence between morphological and molecular approaches are mostly based on hybridization processes and reticulate ancestry ( van Oppen et al., 2001; Willis et al., 2006; Richards et al., 2008, 2013; Chen et al., 2009). Breeding trials under control conditions in the lab among five sympatric species of the aspera group have shown a large potential for natural hybridization and introgression in Acropora ( van Oppen et al., 2002; Willis et al., 2006). However, natural hybridization is still considered rare and probably not an important mechanism of evolution in Acropora as hybrids have low rates of survivorship ( Isomura, Iwao & Fukami, 2013a, b). Hypotheses regarding homoplasy and convergent evolution are not supported by the fossil data as we observed that the morphologies of some Acropora species remain relatively constant for more than 10 Myr. Even though we cannot fully discard the hypotheses that so far vindicate the molecular conundrum in Acropora , alternative explanations can be explored such as the presence of paralogous genes ( Doyle, 1996; Lee et al., 2007). Other morphological characters should also be incorporated in the analysis, including those related to the budding symmetry that we first explored in this study. Recent studies of other Scleractinian taxa with similar issues have successfully reconciled morphologies and molecules by using new morphological characters and molecular markers ( Benzoni et al., 2010; Budd et al., 2012; Huang et al., 2014; Schmidt-Roach et al., 2014). We expect that the full set of morphological features that we present in this study will allow the integration of the fossil record into comprehensive phylogenetic analyses that give more insights into an understanding of the complex evolutionary history of Acropora .