Victoria L. Vance and Dr. Duke S. Rogers, Integrative Biology
The number of species described in the world greatly underestimates its biodiversity, especially in some of the areas more rich in diversity such as Mexico. Once the biodiversity in a region is understood, the issues of management and conservation can be addressed. Spiny pocket mice of the genus Liomys are members of the rodent family Heteromyidae and are grouped with the genus Heteromys in the subfamily Heteromyinae. The genus Liomys has a rather broad geographic distribution that extends from Texas to Panama and consists of five recognized species: L. irroratus, L. pictus, L. spectabilis, L. salvini, and L. adespersus.
Previous research done on Liomys by Genoways (1973) who looked at morphological and chromosomal data proposed that these species were related as follows: L. pictus and L. spectabilis formed the pictus group, L. salvini and L. adespersus formed the salvini group, and L. irroratus formed the irroratus group. Rogers (1990) confirmed this relationship using data based on allozymes. He also noticed that L. pictus, as it was constituted, was paraphyletic which means that not all populations of L. pictus form a single (monophyletic) group relative to L. spectabilis. After further study of allozymes within the pictus species-group, Rogers and Engstrom (1992) determined that there was evidence to suggest that there was more than one distinct biological species within L. pictus.
Before this point, no DNA sequence data have been available to address the question of relationships among species in the genus Liomys. The purpose of my study was to test previous summaries of phylogenetic relationships using sequence data from the mitochondrial-DNA (mtDNA) cytochrome b (cyt b) gene for 70 specimens and from 15 localities. In addition, I tested the proposal of Rogers and Engstrom (1992), that there is more than one species in the L. pictus species group. My research focused on the following questions: how many candidate species exist within the genus Liomys and what are the relationships among these entities? I extracted mtDNA from the liver of my specimens and through standard procedure isolated the cyt b gene and sequenced it. My analyses were done on the computer using a program called PAUP which analyzed the sequenced data based on maximum parsimony criteria and calculated the genetic distances (i.e. the amount of similarity) between populations. The results produced the tree of relationships seen in Figure 1. The average genetic distance within L. salvini is consistent with the values accepted for within subspecies comparisons. Within L. irroratus, however, the average genetic distances are greater than values usually observed within a single species and significantly higher values exist in L. pictus as well. This gives some indication of the possibility of candidate species within the currently recognized groupings.
Further indications of these candidate species can be seen by looking at the population from Jalcocotan which fell into two distinct groups (Fig 1). The genetic distance between the two Jalcocotan populations is, well above the values which normally indicate populations are within the same subspecies. L. p. plantinarensis is the subspecies from Contla, Jalisco and it falls into a sister relationship with L. spectablis (Fig 1), which has been characterized as a distinct species based on differences in morphology, karyotype (Genoways 1973), and allozymes (Rogers 1990) though they are found in the same area together. Again suggesting that some of the subspecies within L. pictus should be considered candidate species.
The mtDNA sequence data indicate that the species pairs L. salvini – L. adespersus and L. pictus and L. spectablis are closely related. These results are consistent with those of Genoways (1973) and Rogers (1990) based on biochemical, chromosomal, and morphological evidence. Thus the division of the current genus Liomys into the three lineages: L. irroratus, (L. salvini + L. adespersus), and (L. pictus and L. spectablis) is still valid. However, the mtDNA data also suggest that Liomys is paraphyletic in relation to Heteromys. Heteromys is more closely related to L. irroratus, and the L. pictus group than it is to the L. salvini group.
Based on the mtDNA sequence data, the current distinction between Heteromys and Liomys as separate genera should be discontinued if verified by additional unlinked markers. The taxonomy within Liomys probably should undergo a revision since there exists four candidate species among the subspecies. Before changes are made, additional sampling of all recognized subspecies of L. irroratus as well as additional geographic sampling of the area between Ameca, Jalisco, and Patzcauro, Michoacán, Mexico is needed in order to delineate the distribution of the sample from Michoacán, Mexico. Also additional sampling throughout the range of L. pictus is needed to obtain a better estimate of the phylogeographic pattern present in this species complex.