Download the inheritance of autosomal and sex

J OURNAL OF C RUSTACEAN B IOLOGY, 34(4), 460-466, 2014
THE INHERITANCE OF AUTOSOMAL AND SEX-LINKED CUTICULAR
PIGMENTATION PATTERNS IN THE MARINE ISOPOD, PARACERCEIS SCULPTA
HOLMES, 1904 (ISOPODA: SPHAEROMATIDAE)
Stephen M. Shuster ∗ , Saundra J. Embry, Carla R. Hargis, and Adrianna Nimer
Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011-5640, USA
ABSTRACT
Cuticular pigmentation is highly variable in Paracerceis sculpta Holmes, 1904, a Gulf of California isopod. Individuals expressing L2r
(= Laterals-2 red) have red pigmentation on the lateral margins of their 6-7th body segments. Individuals expressing 3rs (= Three red
stripes) have three red pigmentation zones running the length of their bodies. Individuals expressing Cd (= Cephalon dark) have black
head capsules. In 12 years of field collections, L2r and 3rs represented less than 0.3% of all individuals (N = 5491) with the frequency
of each sex proportional to population frequencies. Of all individuals scored as Cd (N = 178), 98% were females. We crossed marked
and unmarked parents in all possible combinations. Progeny ratios for L2r and 3rs met Mendelian expectations within families as did
adult expression of 3rs despite observed biases in family sex ratio. In three generations, Cd females crossed to unmarked males produced
families with 1:1 sex ratios, 100% Cd daughters and no Cd sons. Sons from these families never produced Cd daughters. Our results suggest
that L2r and 3rs are controlled by dominant, autosomal alleles. The sex-limited expression of Cd corroborates previous results suggesting
female heterogamety in P. sculpta and in other flabelliferan isopods. The appearance of rare phenotypes controlled by dominant alleles is
paradoxical given the hypothesis that allelic dominance evolves in response to positive selection. However, this combination might persist
if apostatic selection imposed by visual predators occurs in this species’ structured populations, thereby favoring dominance modifiers that
suppress fitness losses by heterozygotes. This species with its large number of cuticular markers could provide tests of this hypothesis.
K EY W ORDS: allelic dominance, apostatic selection, color patterns, female heterogamety, genetics
DOI: 10.1163/1937240X-00002251
I NTRODUCTION
Cuticular pigmentation patterns in isopod crustaceans are diverse and variable (Legrand-Hamelin, 1976; Heath, 1979;
Shuster, 1989). Most patterns appear to be controlled by
dominant Mendelian alleles at autosomal loci, which persist at low frequency in natural populations. Four specific
predictions follow from these results: 1) most individuals bearing cuticular markers in nature are expected to be
heterozygous at the marker locus; 2) both males and females are expected to bear these markers in equal frequency;
3) marked individuals are expected to produce 1:1 ratios of
marked:unmarked progeny when crossed to unmarked individuals; and 4) marked individuals are expected to produce 3:1 ratios of marked:unmarked progeny when crossed
to other individuals bearing the same marker.
Certain other cuticular patterns in isopod crustaceans appear to be sex linked (Legrand-Hamelin, 1976; Shuster
and Levy, 1999). Crustacean sex determination mechanisms
are known to involve combinations of allelic, chromosomal and extrachromosomal factors (Ginsberger-Vogel and
Charniaux-Cotton, 1982; Bull, 1983; Legrand et al., 1987;
Heath and Ratford, 1990; Juchault et al., 1992; Rousset et
al., 1992; Hurst, 1993; Rigaud and Juchault, 1993; Juchault
and Rigaud, 1995). Unless sex change is possible (Shuster and Sassaman, 1997) only members of the heterogametic sex are expected to express the marker when cuti∗ Corresponding
cular marker loci are located on heterochromosomes influencing sex determination. Most studies demonstrating chromosomal sex determination in isopods suggest that females
are the heterogametic sex (ZW = females; ZZ = males;
Legrand-Hamelin, 1976; Ginsberger-Vogel and CharniauxCotton, 1982; Legrand et al., 1987; Juchault and Rigaud,
1995; Shuster and Levy, 1999; although see Tomaszkiewicz
et al., 2010). The sex-limited expression of cuticular pigmentation patterns has been documented in three genera of
flabelliferan isopods to date (Legrand et al., 1987; Shuster
and Levy, 1999).
Paracerceis sculpta Holmes, 1904 is a sphaeromatid isopod native to the central Pacific coast of North America,
with well-studied populations inhabiting the Texas coast of
the Gulf of México (Munguia and Shuster, 2013) and the
northern Gulf of California in the Republic of México (Shuster et al., 2001). Previous studies of this latter population
have documented the inheritance of three autosomal cuticular pigmentation markers (Shuster, 1989) and one cuticular
marker whose inheritance is consistent with sex linkage and
female heterogamety (Shuster and Sassaman, 1997; Shuster and Levy, 1999). In this paper, we document the inheritance of three additional cuticular markers in this species.
Individuals bearing L2r (= Laterals-2 red), have red pigmentation on the lateral margins of each of their sixth and
seventh body segments (Fig. 1a-c). Individuals bearing 3rs
author; e-mail: [email protected]
© The Crustacean Society, 2014. Published by Brill NV, Leiden
DOI:10.1163/1937240X-00002251
SHUSTER ET AL.: INHERITANCE OF CUTICLE PATTERNS IN PARACERCEIS
461
Fig. 1. a, dorsal view of female Paracerceis sculpta bearing the autosomal pigmentation pattern L2r (= Laterals-2 red); b, ventral view of female P. sculpta
bearing the autosomal pigmentation pattern L2r; c, dorsal view of female P. sculpta bearing the W-linked pigmentation pattern, Cd (= Cephalon dark).
(= Three red stripes), have three bands of red pigmentation
running the length of their bodies, one located medially and
one on each lateral body margin (Fig. 2). Individuals bearing Cd (= Cephalon dark) have darkly pigmented cephalons
and bright, white bodies (Fig. 1c).
We present five specific results: 1) the population frequencies of L2r and 3rs were consistent with those reported
for heritable pigmentation markers in this, as well as in
other sphaeromatid isopod species (Shuster, 1989; Shuster
and Levy, 1999); 2) Mendelian inheritance was confirmed
among newly-released progeny in L2r and 3rs; 3) expression
of 3rs among adults in laboratory-reared families confirmed
that this locus is autosomal; 4) a sex-bias in the expression
of Cd existed in population samples of P. sculpta collected
over a 12-year period; and 5) inheritance of Cd was limited
to females in three generations of laboratory reared isopods,
indicating that sex determination in P. sculpta involves female heterogamety (ZW = females; ZZ = males) and that
Cd is W-linked.
We also discuss the hypothesis that rare phenotypes controlled by dominant alleles might persist in this population because visual predators impose apostatic selection on
distinctive isopod color patterns (Jormalainen et al., 1995;
Bourguet, 1999; Bond and Kamil, 1998, 2006). Apostatic
selection occurs when predators form search images that
cause distinctive prey to be taken in disproportionate numbers relative to their actual abundance. This condition favors prey phenotypes that do not resemble preferred prey,
until preferred prey become extremely rare. Then, predators
switch to more abundant prey, modifying their search images to match the phenotype of these now preferred individuals. We suggest that apostatic selection, combined with this
species’ structured populations, may favor dominance modifiers that suppress fitness losses by heterozygotes (Wright,
1929; Bourguet, 1999) and so allow distinctive pigmentation polymorphisms to arise and persist at low population
frequencies.
M ATERIALS AND M ETHODS
Field Collections
Fig. 2. Diagrams. a, α-male Paracerceis sculpta bearing the autosomal
pigmentation pattern L2r; b, female P. sculpta bearing the autosomal
pigmentation pattern, 3rs (= Three red stripes); in life, shaded areas appear
bright red (redrawn after Shuster, 1991b).
Isopods were collected from the spongocoels of the intertidal sponge,
Leucetta losangelensis, in the northern Gulf of California between 1985 and
1997. All individuals were sexed, measured to the nearest 0.125 mm and
identified by cuticular pigmentation pattern (Shuster, 1989). We tabulated
these observations by sex and month. We summed all observations, and
using a Chi-squared test (Sokal and Rohlf, 1995), we compared the number
of individuals of each sex bearing L2r, 3rs and Cd with the numbers of
individuals that were unmarked or which bore some marker other than these
markers (indicated ‘+’).
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JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 4, 2014
Laboratory Experiments
Field-collected females bearing the above markers were maintained in
seawater and were paired with the individuals bearing the same marker
or with unmarked individuals as described in Shuster and Sassaman
(1997). Progeny were separated from females at parturition and placed
into individual, sterilized glass Petri dishes. Individual isopods were reared
to maturity at 24°C on coralline algae (Amphiroa sp.) and brine shrimp
flakes with seawater changes every four days, as described in Shuster and
Sassaman (1997).
We investigated Mendelian inheritance at L2r and 3rs (Figs. 1a-b,
2) by comparing the frequency of marked and unmarked individuals at
birth, and for 3rs, at maturity, within and among families. All parents of
L2r crosses (N = 8) and F1 parents of 3rs crosses (N = 10) were
field-collected, as were all + males in F2 and F3 crosses (N = 3).
All sires of these F1-3 families were α-males (Shuster, 1987). The sire
and dam of the single 3rs × 3rs cross were also field collected but
this sire was a β-male (Shuster and Sassaman, 1997). Consistent with
the predictions of Mendelian inheritance, in all crosses involving one
marked and one unmarked individual, we expected 1:1 ratios of marked:
unmarked individuals. In all crosses involving two marked individuals, we
expected 3:1 ratios of marked: unmarked progeny. We also investigated the
inheritance of family sex ratio by comparing the number of male and female
individuals within and among families. Consistent with the predictions of
chromosomal sex determination, we expected 1:1 sex ratios in all families.
All comparisons were performed using heterogeneity G-tests (Sokal
and Rohlf, 1995). The additive properties of this test provide a means for
examining four possible sources of deviation of observed from expected
frequencies in genetic hypotheses: 1) Gi measures the deviation within
individual i-th crosses of observed phenotypic frequencies from Mendelian
2
expectation, where Gi = X(df=(a−1);α=0.05)
= 3.84 and a = the number
of classes within each test; 2) GTotal measures the deviation of the sum
of all Gi values from their expected magnitude given
the number of tests
of the genetic hypothesis, where GTotal = GT =
Gi(df=b(a−1)) and
b = the number of tests of the genetic hypothesis; 3) GPooled measures
the deviation of the observed from expected phenotypic frequencies,
when observed frequencies are pooled across all tests of the genetic
hypothesis within an experiment, e.g., across all 3rs crosses, and where
2
GPooled = GP = X(df=(a−1);α=0.05)
; and 4) GHeterogeneity measures
the difference between GT and GP , thereby revealing whether observed
frequencies within individual tests of the genetic hypothesis contribute
disproportionately to the value of GT , and where GHeterogeneity = GH =
2
X(df=(b(a−1)−1);α=0.05)
.
A field-collected, gravid female (Shuster, 1991) bearing Cd (Fig. 1c)
was maintained in seawater until she released her progeny. Progeny were
separated from the female at parturition, placed into individual, sterilized
glass Petri dishes, and reared to maturity as described above.
Three F1 Cd females were crossed to unmarked sires (+; N = 3)
from a laboratory lineage (α-1) that consistently produced families with
1:1 sex ratios. Individuals from this ‘α-1’ lineage were homozygous for
the Amsα allele at Ams (= Alternative mating strategy), an autosomal locus
which controls male maturation rate, male external morphology and male
mating behavior (Shuster and Sassaman, 1997). α-1 individuals were also
homozygous for the Tfr1 allele at Tfr (= Transformer), another autosomal
locus that causes sex reversal, depending on an individual’s genotype at
Ams and at primary sex determination loci (Shuster et al., 2001). Since
Amsα and Tfr1 alleles do not interact, the use of Amsα Amsα , Tfr1 Tfr1 sires
in this and in subsequent crosses (see below) minimized the possibility
of sex ratio distortion within families. The F2 generation was reared to
maturity as described above. Four F2 females were crossed to unmarked α1 sires, seven F2 males were crossed to unmarked females from laboratory
stocks, and the F3 generation was reared to maturity as well. Within each
generation, as in all other crosses, all Cd individuals were recorded at
birth as well as at maturity, and all surviving individuals were measured
and sexed, allowing us to determine whether sex- or phenotype-specific
mortality had occurred before maturity.
We investigated Mendelian inheritance of Cd by comparing the frequency of marked and unmarked individuals at birth and at maturity, within
and among families. In all comparisons, we expected 1:1 ratios of marked:
unmarked individuals. We also investigated the inheritance of family sex
ratio by comparing the number of male and female individuals within and
among families. Under chromosomal sex determination, we expected 1:1
sex ratios in all families. All comparisons were performed using heterogeneity G-tests (Sokal and Rohlf, 1995).
R ESULTS
Field Collections
In monthly samples collected over a 12-year period, the
frequency of L2r in the northern Gulf of California P. sculpta
population never exceeded 0.01% within one month, and
was less than 0.001 overall (N = 5491). Over the same
period, the frequency of 3rs in the P. sculpta population
never exceeded 1% within one month, and was less than
0.003 overall. Similar, low but persistent frequencies of
cuticular pigmentation markers have been reported in this
and in other isopod species (Heath, 1979; Shuster, 1989).
In monthly samples collected over the same period, the
frequency of Cd in the northern Gulf of California P. sculpta
population never exceeded 8% within one month, and was
less than 0.02 overall (mean ± 95% CI = 0.015 ± 0.006,
N = 5491). Ninety-eight percent of all Cd individuals
collected were females (N = 178; G = 124.1, P <
0.001, N = 5491), indicating a significant sex-bias in the
expression of Cd in nature. Similar sex-biases in cuticular
marker expression are reported in Idotea, Dynamene and
Paracerceis (Legrand et al., 1987; Shuster and Levy, 1999).
Laboratory Experiments
The frequency of L2r among the progeny of four sets
of field-collected parents met Mendelian expectations (Table 1). Among the three crosses involving one marked and
one unmarked parent, both marked males and marked females produced
similar 1:1 ratios of marked and unmarked
progeny ( Gi(df=3) = GTotal(df=3) = 0.71, P > 0.1;
GPooled = GP = 0.54, P > 0.1, GHeterogeneity(df=2) =
GH(df=2) = 0.17, P > 0.10, Table 1). The 3:1 ratio of
marked:unmarked progeny in the single cross involving two
L2r parents was consistent with Mendelian expectations
(G = 0.09, P > 0.1, Table 1).
The frequencies of marked and unmarked progeny among
all seven crosses involving one parent bearing 3rs and one
unmarked parent also conformed to Mendelian expectations.
(GT(df=7) = 6.93, P > 0.10, N = 438; Table 2). As
with L2r, marked parents of either sex produced similar
progeny ratios (GH(df=6) = 5.61, P > 0.1, Table 2).
The ratios of marked and unmarked progeny among the
six families reared to adulthood were also consistent with
Table 1. Heterogeneity G-tests for Mendelian inheritance of L2r in
Paracerceis sculpta. 1 GT(df=3) = 0.71, P > 0.10; GP = 0.54, P > 0.10,
GH(df=2) = 0.17, P > 0.10. 2 Comparison of observed and expected ratios
of L2r: + at birth; (Ho = 3:1), G(df=1,α=0.05) = 3.84; P > 0.10.
Parental genotype
At birth
Gi
Sire
Dam
L2r
+
N
L2r
L2r
+
Total
+
+
L2r
10
25
14
49
10
21
11
42
20
46
25
91
0.00
0.35
0.36
0.711
L2r
L2r
41
15
56
0.092
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SHUSTER ET AL.: INHERITANCE OF CUTICLE PATTERNS IN PARACERCEIS
Table 2. Heterogeneity G-tests for Mendelian inheritance of 3rs in Paracerceis sculpta (F1 -F3 ). 1 Comparison of observed and expected ratios of 3rs:+ at
birth within families; (Ho = 1:1), G(df=1,α=0.05) = 3.84. 2 Comparison of observed and expected ratios of 3rs:+ at maturity within families; (Ho = 1:1),
G(df=1,α=0.05) = 3.84. 3 Comparison of observed and expected sex ratios within families; (Ho = 1:1), Gi(SR)(df=1,α=0.05) = 3.84. 4 Heterogeneity analysis
of at birth ratios of 3rs:+ for crosses involving one marked and one unmarked parent: GT(df=7) = 6.93, P > 0.10, GP = 1.32, P > 0.10, GH(df=6) = 5.61,
P > 0.10. 5 Heterogeneity analysis of at maturity ratios of 3rs:+ for crosses involving one marked and one unmarked parent; GT(df=6) = 2.92, P > 0.50,
GP = 1.96, P > 0.10, GH(df=5) = 0.96, P > 0.90. 6 Heterogeneity analysis of sex ratio; GT(SR)(df=6) = 15.29, P < 0.025, GP(SR) = 0.60, P > 0.10,
GH(SR)(df=5) = 14.69, P < 0.025; with first F2 cross excluded, GT(SR)(df=5) = 8.43, P > 0.10, GP(SR) = 0.01, P > 0.90, GH(SR)(df=4) = 8.42,
P > 0.10. 7 Comparison of observed and expected ratios of 3rs:+ at birth within families; (Ho = 3:1), G(df=1,α=0.05) = 3.84. ∗ P < 0.05.
Generation
Genotype
Sire
F1
F2
F3
Total
F1
At birth
Dam
3rs
+
At maturity
G1i
N
+
3rs
F
M
F
M
N
G2i
G3i(SR)
3rs
+
3rs
3rs
3rs
+
+
+
+
3rs
+
+
+
3rs
3rs
3rs
11
54
–
80
41
6
8
7
207
12
44
–
94
51
9
16
5
231
23
98
–
174
92
15
24
12
438
0.04
1.02
–
1.13
1.09
0.60
2.72
0.33
6.934
–
–
6
24
9
5
0
3
47
–
–
2
12
6
1
5
1
27
–
–
6
9
9
6
9
2
41
–
–
4
38
6
1
0
2
51
–
–
18
83
30
13
14
8
166
–
–
0.22
1.46
0.00
0.08
1.16
0.00
2.925
–
–
2.04
3.51
1.21
6.86∗
1.16
0.51
15.29∗,6
3rs
3rs
94
22
116
2.39
15
28
2
8
52
3.15
8.98∗
Mendelian inheritance ( Gi(df=7) = 2.92, P > 0.50;
GP = 1.96, P > 0.10; GH(df=6) = 0.96, P > 0.90;
Table 2), despite the appearance of deviations in family sex
ratio ( Gi(SR)(df=6) = 15.29, P < 0.025, GP(SR) = 0.60,
P > 0.10, GH(SR)(df=5) = 14.69, P < 0.025). With the first
F2 cross excluded (see Discussion), no sex ratio deviation
was apparent (GiSR(df=5) = 8.43, P > 0.10, GP(SR) = 0.01,
P > 0.90, GH(SR)(df=4) = 8.42, P > 0.10). The deviant
cross showed a significant excess of females (85%).
We observed no significant deviation from 3:1 Mendelian
expectations in the frequencies of 3rs:+ progeny in the
single 3rs × 3rs cross, either at birth (G = 2.39, P > 0.10,
N = 116) or at maturity (G = 3.15, P > 0.05, N = 52).
However, this family showed a significant excess of males
(69%; G = 8.98, P < 0.01, N = 52).
Although the frequency of Cd among the progeny of
the field-collected parental female did not meet Mendelian
expectations due to an excess of Cd progeny (G = 16.98,
P < 0.001, N = 67; Table 3), the adult ratio of Cd and +
progeny within this family was consistent with Mendelian
inheritance (G = 0.33, P > 0.95, N = 12). Moreover, in
the two generations in which Cd females descended from the
field-collected Cd female were crossed to unmarked males,
Cd showed Mendelian inheritance at birth within
and among
all families (with the F1 family excluded:
Gi(df=7) =
2.62, P > 0.90; GP = 0.05, P > 0.50; GH(df=6) = 2.57,
P > 0.90; Table 3).
Table 3. Heterogeneity G-tests for Mendelian inheritance of Cd and sex ratio in Paracerceis sculpta (F1 -F3 ). 1 P < 0.001. 2 All crosses included:
GP = 3.64, P > 0.05, GH(df=7) = 15.96, P < 0.05. 3 F1 at birth ratio excluded: GP = 0.05, P > 0.50; GH(df=6) = 2.57, P > 0.50. 4 GP = 0.10,
P > 0.50, GH(df=7) = 2.92, P > 0.90.
Generation
At birth
Cd
F1
F2
F3
+
At maturity
N
+
Cd
Gi
Sex ratio/Cd
F
M
F
M
N
Gi
50
11
9
11
15
18
8
17
17
17
8
10
10
19
9
19
67
28
17
21
25
37
17
36
16.981
1.30
0.06
0.05
1.01
0.03
0.06
0.11
7
7
4
5
2
9
2
9
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
5
6
7
2
2
11
2
7
12
13
11
7
4
20
4
16
0.33
0.08
0.83
1.33
0.00
0.20
0.00
0.25
139
89
109
92
248
181
19.602
2.623
45
0
0
42
87
3.024
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JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 34, NO. 4, 2014
Among adults, both Cd and family sex ratio showed
Mendelian
inheritance within and among all families
( Gi(df=8) = 3.02, P > 0.90; GP = 0.10, P > 0.50;
GH(df=7) = 2.92, P > 0.90; Table 3). All females expressed
Cd, whereas all males were unmarked, indicating close linkage between Cd and sex (Exact X 2 probability = 2.55 ×
10−19 , N = 87). This result casts doubt on the hypothesis that Cd is a recessive Z-linked marker. Females bearing
such a marker would have produced all unmarked progeny
in both sexes. This was never the case in 8 crosses. Moreover, unmarked F2 males (N = 7) crossed to unmarked females produced no Cd progeny of either sex (N = 258), indicating that Cd is not a recessive allele transmitted through
male lineages. These results are consistent with Mendelian
inheritance of Cd and with chromosomal sex determination
involving female heterogamety in this species.
D ISCUSSION
Our results suggest that 3rs and L2r in P. sculpta are
autosomal traits and are consistent with reports of the
inheritance of cuticular pigmentation patterns in this, as well
as in two other species of flabelliferan isopods (reviews in
Ginsberger-Vogel and Charniaux-Cotton, 1982; Legrand et
al., 1987). Our results also indicate that Cd in P. sculpta
is a sex-linked trait. In particular, the appearance of Cd in
three consecutive generations of daughters, never in sons,
and never among the progeny of F2 sons of Cd mothers,
indicates that females in this species are heterogametic, and
that Cd is located on the W-chromosome (ZW = female;
ZZ = male).
The appearance of sex ratio biases within certain families
(Table 2) is consistent with other results indicating that
female biased sex ratios occur primarily in families sired
by α-males, whereas male biased sex ratios occur primarily
in families sired by β-males (Shuster and Sassaman, 1997;
Shuster et al., 2001). Sex ratio biases of this sort appear
to be caused by interactions between the primary sex
determination mechanism and two other autosomal loci
(Ams and Tfr; Shuster and Sassaman, 1997) in P. sculpta.
Note that despite the observed sex ratio biases, the ratios of
3rs:+ individuals met Mendelian expectations.
The decreased fecundity of F2-3 families compared to
F1 families is explained by the negative relationship between body size and fecundity in P. sculpta (Shuster, 1991).
Isopods maintained in incubators at 24°C (F2-3 ) were smaller
in size and less fecund than the field-collected parental Cdfemale, who had matured at a cooler sea surface temperature (17-20°C in March; Shuster and Guthrie, 1999). Despite
these differences in fecundity among generations, no differential mortality was detectable between Cd and non-Cd individuals, or between males and females (Table 3). Although
genetic factors are known to cause reversal of sexual phenotype in P. sculpta and in other isopods (Legrand et al., 1987;
Juchault and Rigaud, 1995; Shuster and Sassaman, 1997),
our use of α-sires from families with unbiased sex ratios (lineage α-1) and the lack of biased sex ratios within and among
our F1-3 Cd families indicate that factors responsible for sex
reversal were not present in these crosses.
The excess of Cd progeny among the progeny of the fieldcollected parental female, followed by Mendelian inheri-
tance and sex-linkage of Cd among her progeny and grandprogeny, could be explained if this female had been mated
in the field by an α-male bearing Cpd (= Cephalon, pleotelson dark; Shuster, unpublished data). Individuals bearing
this cuticular pattern possess cephalons as well as pleotelsons that are both darkly pigmented. Newly released Cpdbearing mancas (juveniles) are sometimes difficult to distinguish from Cd mancas, although Cd and Cpd are usually
distinct in laboratory-reared adults. However, field-collected
α-males bearing Cpd can sometimes appear to bear only Cd,
when the pleotelsons of older α-males become encrusted
with bacterial plaques (Shuster, 1987). The appearance of
three Cd males (0.0006, N = 5491) among field collected
individuals may be due to such misdiagnoses.
The cuticular pigmentation markers within the Isopoda,
whose mode of inheritance is known, are controlled by
dominant alleles at Mendelian loci and are observed within
natural populations at low population frequencies (LegrandHamelin, 1976; Shuster, 1989; Shuster and Levy, 1999).
Because these markers appear to be conspicuous to visual
predators such as fish, their low population frequency
has been attributed to directional selection (Jormalainen
et al., 1989), and also to environmental factors such as
salinity or temperature which appear to select against these
patterns (Heath, 1979; Khazaeli and Heath, 1979). However,
this explanation alone is at odds with the hypothesis that
dominance evolves because such alleles are favored by
selection (Nachman, 2005). The possibility exists that these
markers are associated with metabolic pathways whose
kinetic structure leads to marker expression as a dominant
trait (Bourguet, 1999), or that past selection has favored an
excess of enzymatic activity in response to unpredictable
environmental fluctuation (Wright, 1929; Haldane, 1930;
Orr, 1991; Forsdyke, 1994). However, another possible
explanation for the unusual observed combination in these
species could be that these predators impose apostatic
selection on isopod pigmentation patterns.
Apostatic selection, a form of negative frequency-dependent selection, occurs when visual predators form search
images on distinctive prey phenotypes (Allen and Clarke,
1968). Predators continue to seek preferred prey phenotypes
even after they become rare, allowing prey bearing other
phenotypes to increase in frequency due to their higher fitness relative to preferred prey. However, when preferred
prey become extremely rare, predators shift to more common prey, in turn forming search images on them, which relaxes selection against the previously preferred prey phenotype and intensifies it against the new prey phenotype. Such
selection allows multiple prey phenotypes to persist in nature
at comparatively low frequency, and also is thought to cause
prey phenotypes to undergo periodic frequency oscillations
(Cook, 2005), a combination of population-wide phenomena
that is well documented in isopod populations bearing polymorphic cuticular markers (Shuster, 1989; Isocladus sp., J.
Dale, personal communication).
Bond and Kamil (2006) showed, using computer images
of moths that were preyed upon by blue jays, that when moth
phenotypes were variable and were allowed to evolve in response to predation, jays were less likely to detect atypical moths, particularly if these moths were cryptic when
SHUSTER ET AL.: INHERITANCE OF CUTICLE PATTERNS IN PARACERCEIS
viewed against a complex background. Furthermore, this
tendency resulted in apostatic selection imposed by birds
on moth populations even under conditions of high moth
variability, and was not necessarily restricted to the maintenance of a limited number of discrete morphs, e.g., in classic cases of apostatic selection as is observed in land snails
(Clark, 2005), stick insects (Sandoval, 1994) and water boatmen (Popham, 1941). Evolving moths showed significantly
greater phenotypic variance than that of unselected controls
over successive generations, indicating that apostatic selection can encourage the evolution of phenotypic diversity, not
simply dimorphism.
Wright (1929) argued that while the intensity of selection
on dominant mutant alleles can only be on the order of the
mutation rate itself, even weak selection can favor alleles
that modify the expression of rare dominant alleles and cause
both alleles to increase in frequency. Such modifiers appear
to be especially effective when balanced polymorphisms
exist and when populations are spatially structured (review
in Bourguet, 1999). Isopod populations with polymorphic
pigmentation patterns do appear to experience apostatic
selection from predation by fish (Jormalainen et al., 1995;
Maskell et al., 1977). Furthermore, there is evidence that
limited dispersal by P. sculpta mancas may cause significant
population structure in this species over relatively short
(<1 km) spatial scales (Johnson and Shuster, 1999).
If selection on pigmentation patterns in P. sculpta is indeed apostatic as described above, and if such selection can
increase the frequency of rare phenotypic variants in visually complex habitats, particularly when prey populations
are structured, then selection on cuticular pigmentation variants, perhaps through the influences of modifiers, could not
only favor dominant expression in these markers, but maintain them at low population frequencies as well. Although
Bond and Kamil (1998, 2006) did not explicitly explore the
evolution of dominance in their model, their computer algorithm used haploid chromosomes and so simulated this
phenotypic effect.
Our results provide evidence of Mendelian inheritance of
three additional cuticular pigmentation patterns in P. sculpta,
they support existing evidence of chromosomal sex determination and female heterogamety in this species (Shuster
and Sassaman, 1997; Shuster and Levy, 1999), and they are
consistent with reports of these mechanisms in two other
species of flabelliferan isopods (reviews in Ginsberger-Vogel
and Charniaux-Cotton, 1982; Legrand et al., 1987). Our results also indicate that the three cuticular patterns described
here, like other similar markers described for this species,
are rare within the population and segregate as dominant
alleles. These results suggest that this experimental system
could provide a useful model for examining the evolution of
dominance in a marine species.
ACKNOWLEDGEMENTS
This research was supported by NSF grants OCE-8401067, BSR-8700112,
BSR-9106644, DEB-9726504, the NSF/DOE Research Internships Program in Neural and Behavioral Sciences, and the NSF Undergraduate Research Mentorships Program at Northern Arizona University, grants DBI9988009 and DBI-1041255. Additional support was provided by the Department of Biological Sciences, the Organized Research Program and the
Henry O. Hooper Undergraduate Fellowship program at NAU. Assistance
465
in maintaining laboratory animals was provided by K. Johnson, J. Learned,
E. Omana, K. Perry, K. Ressel, H. Wildey and S. Vuturo. Permission to
study P. sculpta populations in the Gulf of California was granted by the
Republic of México, permits 412.2.1.3.0.2315, A00-702-06296 and DAN
02384.
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R ECEIVED: 7 February 2014.
ACCEPTED: 15 March 2014.