Population genetics of the cave shrimp Typhlatya
Michael Scott Webb
Department of Wildlife and Fisheries Sciences, Texas A&M
College Station, Texas USA
Our knowledge of the stygobitic (aquatic,
cave-limited) fauna from the Yucatan Peninsula, Mexico is still in an
early stage. All previously published taxonomic descriptions of the fauna
have been based strictly on morphological characteristics. However,
fundamental questions pertaining to systematics, biogeography,
evolutionary origins and population dynamics can be best addressed with
modern genetic techniques such as DNA sequencing of nuclear and
mitochondrial genes. The purpose of this study is to determine the
intraspecific population structure for the shrimp Typhlatya mitchelli
(Fig. 1) found in the anchialine
caves of Quintana Roo and Yucatan, Mexico by sequencing fragments of
multiple genes. This widespread stygobitic genus contains 10 described
species from fresh and salt water caves in the Atlantic, Mediterranean and
western Pacific (Fig. 2).
Mitochondrial genes including cytochrome b, cytochrome oxidase I,
and 16S rRNA were sequenced (Fig.
3), since they have proven useful in other crustacean population
studies (Palumbi & Benzie, 1991; Knowlton et al., 1993; Machado et al.,
1993). Genomic DNA provided the starting template for all PCR reactions.
Shrimp specimens from multiple cave systems across the Yucatan Peninsula (Fig. 4) were collected and genetic
variability within and among these populations was assessed to test the
effects of geographic distribution on genetic relatedness. Typhlatya
pearsei was used as an outlier group because it too inhabits caves and
cenotes throughout the Yucatan Peninsula and is believed to have colonized
caves at about the same time as T. mitchelli (Wilkens, 1982). All
three genes sequenced show very little divergence, either within
population or across the entire study area. These results lead to several
possibilities: 1) the selected genes do not change over time, 2) these
shrimp recently colonized these caves and have not had sufficient time to
diverge genetically, or 3) there is gene flow across the Yucatan through
an extensive network of underground cave systems. The large number of
published studies on these mitochondrial genes and their frequent use in
population studies should rule out the first possibility. An unusually
high number of relict taxa are found in anchialine caves worldwide
(Iliffe, 2000), including the Yucatan, suggesting that these caves have
been inhabited for a long period of time and therefore ruling out the
second option. Possibility number three seems to be the best explanation
for the data and the sequencing of mitochondrial genes from additional
stygobitic taxa (e.g., amphipods, mysids, isopods, copepods or other
shrimps) could strengthen this hypothesis.
|Figure 1: Typhlatya mitchelli
||Figure 2: Geographic distribution
of species within the shrimp genus Typhlatya
|Figure 3: Portion of DNA sequence
chromatogram using BioEdit software (Hall, 1999).
||Figure 4: Geographic distribution
of shrimp specimens used in this study
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