Publication: Georgia Journal of Science
Publication Date: 22-DEC-05
Delivery: Immediate Online Access
Author: Flowers, Melody A. ; Fang, Quentin Q. ; Pung, Oscar J.
Article Excerpt
ABSTRACT
The grass shrimp Palaemonetes pugio is one of the most abundant macroinvertebrates in southeast coastal marshes but little is known about the genetic structure of this crustacean. Our objective was to determine the amount of genetic diversity of P. pugio populations from the southeast U.S.A. based on geographic location and on the presence of a common grass shrimp parasite, the trematode Microphallus turgidus. Palaemonetes pugio shrimp were collected at 10 localities in South Carolina, Florida and Georgia. Shrimp DNA was extracted and mitochondrial 16S rDNA amplified and then studied using single-strand conformation polymorphism. We found that P. pugio populations are highly conserved among localities suggesting a significant amount of gene flow. There was no correlation between the most common shrimp haplotypes and parasite density or prevalence. Key words: Palaemonetes pugio, Microphallus turgidus, single-strand conformation polymorphism, gene flow
INTRODUCTION
The grass shrimp Palaemonetes pugio is one of the most abundant macroinvertebrates in tidal marshes along the north Atlantic and Gulf coasts of the U.S.A. (1). This small decapod crustacean is an omnivorous deposit feeder and an important source of food for carnivorous fish, birds, mammals and larger invertebrates (1). Consequently, P. pugio is a crucial link in estuarine food chains. However, little is known about the population genetics of this shrimp. Such studies determine the distribution of alleles within and among populations and are important because they provide knowledge of evolutionary forces acting on those populations such as gene flow, genetic drift, mutation, natural selection and speciation (2). A goal of our project was to perform a preliminary examination of the genetic structure of P. pugio populations collected from localities on the southeast coast. Because coastal populations of grass shrimp are probably not completely isolated from each other, we predicted that we would find significant gene flow among populations and that genetic distance would not increase with geographic distance.
Parasitic organisms may play a role in the evolution of their hosts and are thought to generate and maintain host genetic polymorphism (3). However, studies demonstrating that parasites exert selective pressure on their hosts are lacking. The second goal of our study was to determine if a common grass shrimp parasite, the trematode Microphallus turgidus, affects the genetic variability of P. pugio populations. Based on the studies of Lively and Dybdahl (4), who observed that clonal snail populations with common haplotypes were more heavily infected with trematodes than rare snail haplotypes, we hypothesized that P. pugio populations with high parasitism would also have the most common haplotype.
MATERIALS AND METHODS
Dip nets were used to collect P. pugio grass shrimp from creeks or rivers adjacent to salt marshes at 10 localities on the coasts of South Carolina, Georgia and Florida from October 2001 to July 2002 (Table I, Figure 1). The shrimp were transported to the laboratory in aerated water from the collection site and then identified using morphological features described by Anderson (5), measured and weighed. A dissecting microscope (10X total magnification) was used to count metacercarial cysts of the parasite M. turgidus. The prevalence (i.e., the percent of grass shrimp infected) and density (i.e., number of metacercarial cysts per cm grass shrimp body length) of the parasite were calculated. Shrimp were then killed by freezing and stored at -80[degrees]C until needed.
[FIGURE 1 OMITTED]
DNA was extracted from abdominal muscle of individual shrimp using the protocol of Doyle and Doyle (6) as modified by Mixson et al. (7). Primers for polymerase chain reaction (PCR) amplification of...
NOTE: All illustrations and photos have been removed from this article.
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