S.S. Taylor, S. Woltmann, A. Rodriguez, and W.E. Kelso
2013 Southeastern Naturalist Vol. 12, No. 3
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2013 SOUTHEASTERN NATURALIST 12(3):514–522
Hybridization of White, Yellow, and Striped Bass in the
Toledo Bend Reservoir
Sabrina S. Taylor1,*, Stefan Woltmann1, Andrew Rodriguez1,
and William E. Kelso1
Abstract - Long-term stocking of non-native Morone saxatilis (Striped Bass) in Toledo
Bend Reservoir may have had adverse effects on the integrity of the native Morone chrysops
(White Bass) and M. mississipiensis (Yellow Bass) genome through introgression. We
examined microsatellite genotypes for evidence of hybridization in a sample of Striped,
White, and Yellow Bass but found only four potential hybrids. Despite the introduction of
millions of Striped Bass over four decades, there is no evidence for either a hybrid swarm
or substantial introgression. Low numbers of hybrids may be the result of poor hybrid survival,
little reproduction between species, or a combination of both.
Introduction
The Toledo Bend Reservoir on the border of Louisiana and Texas supports
native populations of Morone chrysops Rafinesque (White Bass) and M. mississippiensis
Jordan and Eigenmann (Yellow Bass), and was stocked from 1967 to 2009
with ≈9.37 million M. saxatilis Walbaum (Striped Bass) to enhance recreational
fishing opportunities. Stocking ceased when surveys indicated a lack of interest
in the fishery. Introduced Striped Bass can hybridize and produce fertile offspring
with White Bass in the wild (Avise and Van Den Avyle 1984). Similarly, introduced
Striped Bass can hybridize with Yellow Bass, producing 100% females in hatchery
crosses (Wolters and DeMay 1996). We were interested in whether the integrity of
the White and Yellow Bass genome had been compromised by introgression over
the past four decades. The introgression of genes from one species into another is a
serious concern because it has caused or contributed to the extinction of many species,
including several fishes (Allendorf and Luikart 2007, Rhymer and Simberloff
1996). Moreover, White and Yellow Bass may waste reproductive effort by mating
with Striped Bass, which can depress population growth rates and decrease population
size (Leary et al. 1993, Rhymer and Simberloff 1996).
From a hatchery perspective, hybridization may be a problem if hybrids cannot
be distinguished from Striped Bass and are subsequently used by hatcheries
as brood stock during annual spawning efforts (Fries and Harvey1989, Woods et
al. 1995). Hatchery operations may also unintentionally release hybrid bass via
drains, which may contribute to the overall numbers of White Bass x Striped Bass
hybrids (B. Reed, Louisiana Department of Wildlife and Fisheries [LDWF], Baton
Rouge, LA, pers. comm.). In Toledo Bend Reservoir, qualitative observations
by the LDWF indicate that putative Striped Bass egg color, egg morphology, and
broodstock size have changed through time (R. Yeldell, LDWF, Anacoco, LA,
1 School of Renewable Natural Resources, Louisiana State University AgCenter, RNR
Building, Baton Rouge, LA 70803. *Corresponding author - staylor@agcenter.lsu.edu.
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2013 Southeastern Naturalist Vol. 12, No. 3
pers. comm.). Furthermore, preliminary allozyme studies indicate that at least
F1 hybrids are present in the reservoir (W.E. Kelso, unpubl. data). Collectively,
these observations suggest that hybrids are present in the reservoir, which may
have ultimately affected the genetic integrity of both native and introduced
Morone species. However, data collected to date cannot quantify the degree
of backcrossing between hybrids and the parental species. To assess whether a
hybrid swarm is present, analyses of multiple, co-dominant loci such as microsatellites
are necessary (Boecklen and Howard 1997).
In this paper, we assess whether long-term stocking of Striped Bass has created
hybrid bass swarms and altered the genetic make-up of native Morone species
in Toledo Bend Reservoir. We also examine whether identification of Striped
Bass and hybrid Striped Bass based on morphological characteristics is reliable;
an important concern related to broodstock collection (Avise and Van Den Avyle
1984, Kerby 1979; Scribner et al. 2000). To address these issues, we present data
on the frequency of genetically identified hybrids observed in a sample from the
reservoir, and whether hybrids were correctly identified in the field.
Methods
White Bass (n = 25), Yellow Bass (n = 20), Striped Bass (n = 27), and morphologically
identified hybrid White Bass x Striped Bass (n = 3) were captured
in 2011 by electrofishing on the Sabine River above the Toledo Bend Reservoir,
or by rod and reel in the Reservoir near Highway 1215. Blood was drawn from
the caudal vein for DNA analysis. Additional whole Yellow Bass (n = 40) were
collected from Poverty Point (Richland Parish, LA), where Striped Bass have
never been present, to obtain Yellow Bass allele frequencies without the potential
influence of Striped Bass hybridization. White Bass and Striped Bass
allele frequencies given in the literature (Couch et al. 2006) were used as reference
data because no pure White or Striped Bass populations exist in Louisiana
as far as we are aware. Blood was stored in Queen’s lysis buffer (Seutin et al.
1991) and whole Poverty Point bass were frozen. DNA from blood or caudal fin
tissue was extracted with the Qiagen DNEasy kit (Valencia, CA). Samples were
genotyped with 14 previously developed microsatellite loci as follows: MSM
1075, 1078, 1097, 1102, 1106, 1107, 1137, 1138, 1144, 1149, 1154, 1157, 1177,
1246 (Couch et al. 2006). These loci had non-overlapping, species-specific allele
size ranges for White and Striped Bass (Couch et al. 2006). DNA was amplified
via polymerase chain reaction (PCR) with 5–10X buffer (New England
Biolabs), 0.16 mM dNTPs (Qiagen), 2.0 mM MgCl2, 0.5 U Taq polymerase
(New England Biolabs), 0.16 μM primers tagged with M13 forward or reverse
tails (Operon), 0.008 μM M13 Forward or Reverse IRDye 700 or 800 (Li-COR
Biosciences), and nanopure water for a total reaction volume of 10 μL. Some
reactions also included 0.5 M betaine and 3% by volume DMSO. Thermocycling
conditions consisted of 2 min at 94 °C followed by 35 cycles of 94 °C for
30 s, 55–61 °C for 30 s, and 72 °C for 30 s with a final extension step of 72 °C
for 7 min. After PCR, 3 μL of stop dye was added to the reactions, followed by
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2013 Southeastern Naturalist Vol. 12, No. 3
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a 4 min denaturation step at 94 °C, and then 0.8 μL of the mixture was electrophoresed
on a Li-COR 4200 automated DNA analyzer with size-standard
IRDyes of 50–350 bp (Li-COR Biosciences). Potential hybrid genotypes were
verified with replicate PCR reactions. Alleles were scored with Saga® software
(v. 3.2; Li-Cor Biosciences), and verified by eye.
Statistical analyses
Allele frequency distributions were calculated with Genetix v. 4.05 (Belkhir et
al. 1999). A bayesian approach was used to examine genetic clustering among the
three species and hybrids based on the program Structure v. 2.3.2 (Pritchard et al.
2000). For this analysis, a burn-in period of 100,000 was used with 100,000 repetitions
for each of five simulations with K = 3 populations. Prior information on
the species identity or source population was excluded in order to obtain results
based solely on the genetic data. Individuals with less than 95% membership in
a species were considered to be hybrids.
Results
Alleles for individual species were largely distinct in Toledo Bend Reservoir
and identification of hybrids was straightforward (Tables 1, 2). Allele frequency
distributions indicated that four hybrids were present among the individuals
sampled (Table 1). One individual was a potential Yellow x Striped Bass hybrid
from Toledo Bend. Four loci did not amplify in this individual, but at the remaining
loci, all alleles were consistent with Yellow Bass allele frequencies except for
one locus that was homozygous for a Striped Bass allele (Table 1). The remaining
three hybrids were White Bass x Striped Bass hybrids that were morphologically
Table 1. Genotypes of the four potential hybrids. Bold formatting indicates genotypes consistent
with F1 hybrids, and underlining indicates genotypes consistent with backcrossing or mating
among hybrids.
Bass species
White x Striped White x Striped White x Striped Yellow x Striped
MSM1075 247219 247219 233219 205205
MSM1078 163147 163145 159147 169169
MSM1102 161145 161145 161145 165165
MSM1137 168140 168140 166140 152152
MSM1138 203183 203181 203181 187187
MSM1154 174000 216174 200174 165165
MSM1149 210198 216198 210198 0
MSM1144 198163 196151 196141 171171
MSM1157 203161 161161 161161 165157
MSM1177 209209 209209 209209 209209
MSM1107 170154 154154 154154 0
MSM1097 160150 166150 166150 0
MSM1106 167165 165165 165165 159159
MSM1246 256214 214000 214214 0
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2013 Southeastern Naturalist Vol. 12, No. 3
Table 2. Bass allele frequencies for 14 microsatellite loci. Potential hybrids (n = 4) are not included
in the allele frequencies given (see Table 2 for hybrid genotypes). Freq. = frequency.
Yellow Bass: Yellow Bass:
White Bass Striped Bass Toledo Bend Poverty Point
Allele Allele Allele Allele
Locus size (bp) Freq. size (bp) Freq. size (bp) Freq. size (bp) Freq.
MSM1075 219 1.0000 225 0.0385 205 1.0000 205 1.0000
227 0.1538
229 0.0577
231 0.0577
233 0.2308
235 0.1346
237 0.0385
239 0.1154
247 0.1538
321 0.0192
MSM1078 159 0.0600 145 0.0926 169 1.0000 163 0.1316
163 0.9400 147 0.9074 169 0.8684
MSM1097 150 1.0000 160 0.3462 182 1.0000 182 1.0000
162 0.1920
164 0.0385
166 0.0385
168 0.5577
MSM1102 145 1.0000 161 1.0000 165 1.0000 165 1.0000
MSM1106 202 0.8571 165 0.3889 159 1.0000 159 1.0000
204 0.1429 167 0.5556
171 0.0556
MSM1107 154 1.0000 170 0.9524 200 1.0000 200 1.0000
172 0.0476
MSM1137 140 1.0000 142 0.4167 150 0.3947 150 0.2375
160 0.0278 152 0.6053 152 0.7625
168 0.1389
170 0.0556
172 0.0833
174 0.2500
181 0.0278
MSM1138 177 0.1000 183 0.1296 187 1.0000 187 1.0000
181 0.8400 203 0.1667
183 0.0600 205 0.1667
207 0.3184
209 0.1667
211 0.0556
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identified in the field as hybrids. These fish had genotypes consistent with F1 offspring
at 9–12 loci (one allele from each parental species) but showed evidence
for backcrossing or mating among hybrids at 2–5 loci (both alleles from a single
parental species; Table 1).
Structure v. 2.3.3 analyses clustered White Bass and Striped Bass as
separate groups with their hybrids as intermediates between the two clusters
(Fig. 1). Yellow Bass from Toledo Bend Reservoir and Poverty Point clustered
Table 2, continued.
Yellow Bass: Yellow Bass:
White Bass Striped Bass Toledo Bend Poverty Point
Allele Allele Allele Allele
Locus size (bp) Freq. size (bp) Freq. size (bp) Freq. size (bp) Freq.
MSM1144 141 0.0208 141 0.3184 171 1.0000 171 1.0000
194 0.0208 147 0.0370
196 0.7292 151 0.1481
198 0.0833 153 0.0185
199 0.0208 157 0.1481
200 0.1250 159 0.1111
161 0.0741
163 0.1111
171 0.0370
MSM1149 198 1.0000 210 0.8704 200 1.0000 200 1.0000
214 0.0185
216 0.1111
MSM1154 174 1.0000 200 0.0370 165 1.0000 165 1.0000
216 0.8148
218 0.1481
MSM1157 161 1.0000 161 0.0400 157 0.6053 157 0.0256
185 0.0200 165 0.3684 165 0.5769
193 0.1000 169 0.0263 169 0.3974
203 0.2000
205 0.1600
211 0.0200
217 0.4200
223 0.0400
MSM1177 237 1.0000 207 0.0962 215 1.0000 215 1.0000
209 0.8846
211 0.0192
MSM1246 214 1.0000 224 0.0185 214 0.0357 226 0.0132
236 0.0926 262 0.9643 262 0.8816
244 0.1111 266 0.1053
256 0.6296
262 0.1481
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2013 Southeastern Naturalist Vol. 12, No. 3
closely together and were proximal to the Yellow x Striped Bass hybrid
(Fig. 1). Structure v. 2.3.3 results giving the proportion of membership in each
of three species are provided in Table 3.
Table 3. Proportion of membership in each of three species based on Structure results. n = number
of individuals.
Inferred cluster
Species or hybrid White Bass Yellow Bass Striped Bass n
White Bass 0.997 0.001 0.002 25
White x Striped Bass 0.484 0.001 0.515 3
Striped Bass 0.001 0.001 0.998 27
Yellow x Striped Bass 0.001 0.903 0.095 1
Yellow Bass Toledo Bend 0.003 0.996 0.001 19
Yellow Bass Poverty Point 0.001 0.998 0.001 40
Figure 1. Analysis of bass genotypes using Structure v. 2.3.2. Black circle = Striped Bass,
grey circle = White Bass, vertically striped circle = White x Striped Bass hybrids, white
circle = Poverty Point Yellow Bass, horizontally striped circle = Toledo Bend Yellow
Bass, spotted circle = Yellow x Striped Bass hybrid.
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Discussion
Despite many years of stocking with Striped Bass, Toledo Bend Reservoir
shows no evidence of hybrid bass swarms despite considerable potential for this
to occur both through hybridization in the wild following the release of millions
of Striped Bass, and inadvertent stocking of hybrids via hatchery escapees. Although
more extensive collections and larger sample sizes may produce different
numbers, our results suggest that, given the small number of hybrids present,
introgression-related impacts on native species’ genomes appear to be minimal.
Of 75 fish sampled, only 4 hybrid individuals were identified, and three of these
were recognized through morphological differences.
Our genetic analyses identified only one Yellow x Striped Bass hybrid, suggesting
that hybridization rates between these species may be low and, therefore,
introduced Striped Bass are not causing introgression in Yellow Bass. Three
White Bass x Striped Bass hybrids were present and had genotypes consistent
with F1 hybrids at 9–12 loci, and genotypes at 2–5 loci that indicated backcrossing
with Striped Bass (one individual at two loci) or crossing among hybrids (two
individuals at four and five loci, respectively). Reproduction between White and
Striped Bass and among Striped Bass hybrids has been documented previously in
several areas including Toledo Bend Reservoir (Avise and Van Den Avyle 1984,
Crawford et al. 1984, Forshage et al. 1986). However, White Bass x Striped Bass
hybrids have a shorter life span and poorer reproductive success than White Bass
(Bartley et al. 2000, Champeau 1984, Ross 2001), and temperatures in the Toledo
Bend Reservoir have caused summer die-offs of large Striped Bass, which do not
tolerate temperatures above 22 °C (Matthews 1985). Together, these factors may
limit the number of White Bass x Striped Bass hybrids in Toledo Bend, especially
following LDWF’s decision to cease stocking efforts in 2009, which eliminated
new inputs of Striped Bass.
The low frequency of bass hybrids in Toledo Bend Reservoir may be attributable
to differences in reproductive strategies and poor hybrid survivorship as
outlined above in published studies. However, other evidence suggests that the
frequency of hybridization may depend on population size. When one species is
abundant and the other is rare, females of the rare species may mate with males
of the abundant species because they do not encounter males of their own species.
For example, Avise and Saunders (1984) reported that genetically identified
sunfish (Lepomis spp.) hybrids were crosses between males of the most common
species (L. macrochirus Rafinesque [Bluegill Sunfish] and L. auritus L.
[Redbreast Sunfish]) and females of the rarest species (L. cyanellus Rafinesque
[Green Sunfish], L. gulosus Cuvier in Cuvier and Valenciennes [Warmouth], and
L. microlophus Günther [Redear Sunfish]). Given that native White and Yellow
Bass populations in the Toledo Bend Reservoir are probably very large, there
may be little risk of females mating with Striped Bass and therefore little risk of
introgression in the native Morone genomes.
Finally, morphological identification of White Bass x Striped Bass hybrids
corresponded to hybrids identified through genetic techniques. These individuals
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2013 Southeastern Naturalist Vol. 12, No. 3
were genetically very similar to F1 hybrids, and may in fact have been F1s if
some loci had null alleles in hybrids (i.e., one species’ allele preferentially amplifies).
Given that the same four loci were often homozygous for parental alleles in
White x Striped Bass hybrids, null alleles are possible.
Although documented morphological differences exist between F1 White x
Striped Bass hybrids and both parental species (Crawford et al. 1984, Kerby
1979), backcrossed individuals or later generations of hybrids may be impossible
to identify with morphological features alone (Avise and Van Den Avyle 1984).
Should hatchery production of Striped Bass resume, we recommend genetic
testing of Striped Bass before choosing brood stock to ensure that the parental
species and not hybrids are propagated.
Acknowledgments
We are very grateful to Ricky Yeldell, Sean Kinney, Debra Kelly, Kelsey Daroca, and
Amanda Bartlett for assistance in the field and the lab. We would like to acknowledge the
Louisiana Department of Wildlife and Fisheries for financial support of this project.
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