Phylogeny
The classification of Xanthorrhoeaceae sensu stricto has in the past included ten genera, listed in the Flora of Australia by Bedford et al. This has been reduced over time to currently just one genus, Xanthorrhoea; while the other nine genera were placed into other families such as Laxmanniaceae and Dasypogonaceae, these remained in the lilioid order Asparagales, along with Xanthorrhoeaceae.
Molecular DNA sequencing has uncovered several monocot clades, although there still remains an unresolved polytomy between three groups: the lilioid orders, the commelinid clade, and the family Petrosaviaceae. The main taxonomic issue within the Asparagales is the placement of Orchidaceae. Recent research that indicates Orchidaceae to be monophyletic. This places it as a sister taxon to all Asparagales, notwithstanding research such as Rudall’s morphological analysis, which suggested a close relationship between Orchidaceae and certain other families within Asparagales, such as Hypoxidaceae.
The order Asparagales is divided into two groups. The lower grade contains early-diverging clades characterised by simultaneous microsporogenesis; and the higher clade is characterised by successive microsporogenesis. The morphological division between successive and simultaneous microsporogenesis is supported by through molecular analysis in comparing monocot rbcL nucleotide sequences, and plastid trnL-F sequences. The Angiosperm Phylogeny Website disputes this finding, claiming that placement based on changes in microsporogenesis is unclear. In the lower Asparagales, which has mostly simultaneous microsporogenesis, the successive type is present in Xanthorrhoeaceae, Hypoxidaceae, Iridaceae and Orchidaceae. This change has been described as a reversal from simultaneous to successive, since this particular change is more frequent across the angiosperms. Even with these findings, simultaneous microsporogenesis continues to be of phylogenic importance within Asparagales. Rudall carried out a morphological cladistic analysis on Asparagales, which included an out-group sample of five families of Liliales. Rudall recognized that restrictions of this type of analysis depends on the broadness of out-group sampling, and the limits of the number of taxa sampled, especially when the lilioid orders have a high homoplasy of floral and vegetative characteristics. Regardless of these sampling problems, Rudall’s morphological analysis indicates that Asparagales has a monophyletic lower clade, rather than a grade. Rudall confirms Chase et al.’s molecular analysis by placing Xanthorrhoeaceae into the lower group in Asparagales, despite sharing the characteristic of successive microsporogenesis with the higher clade. Xanthorrhoeaceae has been found to be closely related to Asphodelaceae and Hemerocallidaceae, with Xanthorrhoeaceae and Asphodelaceae as sister taxa. There is some uncertainty in the branch of Hemerocallidaceae and the Xanthorrhoeaeace/Asphodelaceae branch, reflected in the 50–79% branch support in the phylogenetic relationships within Asparagales. Rudall, in confirmation of this uncertainty, states that by placing Xanthorrhoeaceae in a clade with Asphodelaceae and Hemerocallidaceae, although supported by molecular analysis, is not supported by morphological analysis; however, Rudall does concede that it shares a thickening meristem with Asphodelaceae.
In order to resolve this uncertainty between Xanthorrhoeaceae, Asphodlaceae and Hemerocallidaceae, it has been suggested that the family Xanthorrhoeaceae is to contain a scheme of three subfamilies: Asphodeloideae, Hemerocallidoideae and Xanthorrhoeoideae.
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