Luzon Montane Forest Mouse - Evolution and Phylogenetic Relationships

Evolution and Phylogenetic Relationships

The Luzon montane forest mouse belongs to the Chrotomys division, a group within the Murinae that occurs exclusively on the Philippines, and in addition to Apomys, also includes Rhynchomys, Chrotomys and Archboldomys. Animals in this division share several morphological and genetic features. Within this group, Apomys is by far the biggest and most extensive genus, containing small, inconspicuous wood mice which are common to the whole of the Philippines, while the other, more specialized genera are barely ever found outside Luzon. Apomys itself was divided into two groups, in the aforementioned article by Musser from 1982: the datae group, containing only A. datae, and the abrae-hylocetes group, containing all other species. Animals in these two groups differ in the way in which the head is supplied with blood from arteries. Since the publication of Musser's article, another species has been described that falls into the datae group: A. gracilirostris. This relationship is further supported by other similarities: both species are relatively large for the genus and have a relatively long snout.

In 2003, a phylogenetic study was published which compared DNA sequences from the cytochrome b gene of thirteen species of Apomys. This study confirmed the proposed relationship between the large Mindoro forest mouse (A. gracilirostris) and the Luzon montane forest mouse (A. datae), as well as the status of the datae group as a sister group of the other species of Apomys.

The phylogenetic relationships of the Luzon montane forest mouse can be summarized as follows.

According to this study, the split between the Luzon montane forest mouse and the large Mindoro forest mouse took place some three million years ago, as calculated using a molecular clock, putting it in the pliocene. It was also estimated that Apomys dates back more than four million years, with the Chromotys division being again more ancient by another two million years. Another, more elaborate study concluded that the Chromotys division was older still, at more than ten million years, and that the split between the Chromotys division and its closest relatives, a predominantly African group including Mus, Otomys and Mastomys, took place some sixteen million years ago. Seeing as Apomys probably originated on Luzon, the Luzon montane forest mouse likely developed directly from the ancestor species of the datae group, while the large Mindoro forest mouse is the result of a pliocene migration to Mindoro.