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Natural Selection and Maternal Effects in Life History Traits of Brevicoryne brassicae (Homoptera: Aphididae) on Two Sympatric Closely Related Hosts

Authors: Ruiz-Montoya, Lorena, and Núñez-Farfán, Juan

Source: Florida Entomologist, 92(4) : 635-644

Published By: Florida Entomological Society

URL: https://doi.org/10.1653/024.092.0417

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Ruiz-Montoya & Núñez-Farfán: Host-related Performance of Cabbage Aphid 635

NATURAL SELECTION AND MATERNAL EFFECTS IN LIFE HISTORY TRAITS OF BREVICORYNE BRASSICAE (HOMOPTERA: APHIDIDAE) ON

TWO SYMPATRIC CLOSELY RELATED HOSTS

LORENA RUIZ-MONTOYA1 AND JUAN NÚÑEZ-FARFÁN2 1El Colegio de la Frontera Sur, Carretera Panamericana y Periférico Sur s/n, CP 29290,

San Cristóbal de Las Casas, Chiapas, México E-mail: [email protected]

2Laboratorio de Genética Ecológica y Evolución, Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México. Circuito exterior s/n, Ciudad Universitaria,

CP 04510 Coyoacán, Distrito Federal, México E-mail: [email protected]

ABSTRACT

In this study we used 2 reciprocal transference experiments to estimate the mode and magnitude of selection on life history traits of the aphid Brevicoryne brassicae L. on 2 sympatric host plants, Brassica campestris L. and Brassica oleraceae L. In the first ex- periment, we recorded success in establishment, age at first reproduction, duration of re- production, number of nymphs laid, reproductive rate, and rate of increase (ri) on each host. In the second experiment, the offspring of females collected on Br. campestris or Br. oleraceae were reared on hosts through 3 generations. Age at first reproduction, duration of reproduction, fecundity, and rate of increase (ri) were measured during the third gen- eration in order to determine presence of maternal effects. Within each recipient host, we estimated selection gradients by performing a multiple linear regression of relative number of nymphs in relation to age at first reproduction and duration of reproduction. Results of the first experiment indicate that successful establishment was higher on the source host on which the mother was collected. A significant effect associated with the recipient host was detected for most life history traits. Positive directional selection on duration of reproduction was detected on both hosts whereas selection on the age at first reproduction was detected only in Br. oleraeae. In the second experiment we did not find evidence of maternal effects since the recipient host exerted a significant effect on fecun- dity. Again, the selection on duration of reproduction was detected on both hosts. Despite these results suggesting incipient ecological specialization in B. brassicae, we recognize factors that may prevent the evolution of host plant specialization.

Key Words: host-specialization, local adaptation, phenotypic plasticity, life-history traits

RESUMEN

Usamos dos experimentos de transferencia recíproca para estimar el modo y magnitud de la selección en caracteres de historia de vida de Brevicoryne brassicae L en dos espe- cies huésped simpátricas, Brassica campestris L. y Brassica oleraceae L. En el primer ex- perimento registramos el éxito en el establecimiento, edad a primera reproducción, duración de la reproducción, número de ninfas producidas, tasa reproductiva y tasa de incremento (ri) en cada planta huésped. En el segundo experimento diseñado para medir los efectos maternos, la descendencia de las hembras colectadas en Br. campestris o Br. oleraceae fueron desarrolladas durante dos generaciones en la misma especie de planta en la que fueron colectadas y en la tercera generación se hizo la transferencia recíproca y se midió la edad a la primera reproducción, duración de la reproducción, fecundidad y tasa de incremento (ri). Se estimó la selección natural en cada huésped para la duración y la edad a la primera reproducción mediante regresión lineal múltiple. El número de ninfas depositadas por cada madre se empleó come el estimador del éxito reproductivo. En el primer experimento, el éxito en el establecimiento fue superior en la especie hués- ped donde la madre fue colectada. Hubo un efecto significativo del huésped receptor para la mayoría de las características de historia de vida. Se detectó selección direccional po- sitiva en la duración de la reproducción en ambas especies huésped, mientras que sólo en Br. oleraceae se detectó selección en la edad a la primera reproducción. No hubo evi- dencia de efectos maternos; sólo el huésped receptor tuvo un efecto significativo en la fe- cundidad. Hubo selección direccional en la duración de la reproducción. Aunque los

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636 Florida Entomologist 92(4) December 2009

resultados sugieren especialización ecológica incipiente en B. brassicae, se discuten los factores que pueden limitar la evolución de la especialización de huésped.

Translation by the authors.

Host plants may constitute environments with distinct selection pressures for phytophagous in- sects, resulting in host race formation or host spe- cialization (Diehl & Bush 1984; Farrell 1998; Bush 1994). However, a phenotypic adaptive change in phytophagous insects may be expected whenever phenotypic variations in traits related to host use are heritable and linked to fitness, and when pheno- typic plasticity is negligible (Via & Lande 1985; Scheiner 1993).

In many organisms, environmental effects may be transmitted through generations via mecha- nisms other than transmission of nuclear genes (Rossiter 1987; Mosseau & Dingle 1991; Hunt & Simmons 2000; Agrawal 2001; McAdam et al. 2002). For example, if a phytophagous insect is reared on a given host species, its offspring is “acclimated” to that host, possibly via induction of enzymatic activ- ity in the offspring (Fox et al. 1995; Hunter 2002). This type of inheritance, known as maternal effects, may be relevant for phytophagous insects if more than 1 potential host plant species occur together within a habitat. Maternal effects may influence the evolutionary trajectories of host specialization by preventing selection (Fox & Savalli 2000; Rossiter 1991; McAdam et al. 2002) and may be adaptively important in heterogeneous environments.

Throughout its geographic distribution, the cab- bage aphid, Brevicoryne brassicae L. (Homoptera: Aphididae) uses different Brassicaceae plant spe- cies and displays variability in performance among host plants (Costello & Altieri 1995). Brevicoryne brassicae uses 2 related host plant species that oc- cur sympatrically in Chiapas, Mexico (the weed Brassica campestris L. and the cultivated Br. oler- aceae L.). In Chiapas, B. brassicae reproduces par- thenogenetically and lives from Oct through Mar (L. Ruiz-Montoya, personal observation). Because B. brassicae feeds upon both hosts, its performance on 1 host could be affected if the mother lived on the other host during the previous generation (i.e., ma- ternal effects).

This study assessed whether B. brassicae exhib- its differential performance on Br. campestris and Br. oleraceae, whether selection on life history traits is similar in both hosts, and whether the offspring’s phenotype is influenced by the mother’s environ- ment. Using a reciprocal transference experimental design, we performed 2 experiments in order to es- timate (1) direction and magnitude of phenotypic selection on age at first reproduction and duration of reproduction of B. brassicae on each host plant, and (2) the extent of maternal effects on offspring’s age at first reproduction, duration of reproduction, and fecundity.

MATERIALS AND METHODS

Brevicoryne Biology in the Zone Study

Brevicoryne brassicae is an oligophagous her- bivore which uses Brassicaceae species through- out its worldwide distribution (Blackman & Eas- top 2000). In our study site, B. brassicae feeds upon Br. campestris and Br. oleraceae and has a parthenogenetic life cycle with winged and wing- less morphs. The host plant species, Br. campes- tris and Br. oleraceae, were introduced to Mexico approximately 200 years ago (Rollins 1993). Br. oleraceae (green cabbage) has been cultivated an- nually in the Chiapas Highlands for at least 5 de- cades (Montoya-Gomez 1998). The weed Br. campestris (Syn. Brassica rapa, turnip) is an an- nual herbaceous plant that grows in abandoned fields and on the borders of cultivated Br. oler- aceae fields. As annuals, both host plant species complete their life cycle in 3 to 4 months. Up to 3 generations of the aphid have been recorded in the field during a given growing season.

Brevicoryne Performance and Natural Selection with- out Considering Maternal Effects

Adult wingless aphids from the locality of Balun Canan (16°38’ 19” N latitude; 92°32’ 43” W longitude; 2240m) in the state of Chiapas, Mexico were randomly collected from 30 individual plants of each host species during Mar of 1999. Both host plants were found in the same 4-ha field, and distance among plants ranged from 30- 300 m. Brevicoryne campestris plants were grow- ing adjacent to the cultivated Br. oleraceae fields. In a reciprocal transplant experiment, aphids col- lected in the field from each host plant species (hereafter referred to as source host), were placed singly on individual plants of each host (hereafter referred to as recipient host). Females collected on Br. campestris were transferred to individual plants of Br. campestris (n = 18) or Br. oleraceae (n = 25), whereas aphids derived from Br. oleraceae were placed on Br. oleraceae (n = 22) or Br. campestris (n = 21). Individual plants were ran- domly assigned to greenhouse benches and indi- vidually covered with a rearing chamber made with a fine mesh to exclude other aphids and nat- ural enemies (Blackman 1974). Plants were kept at ambient temperature and photoperiod (13 ± 4°C and 12 h daylight).

Each female collected in the field was allowed to lay a single nymph (1st generation). The life of each nymph was followed until death, recording

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Ruiz-Montoya & Núñez-Farfán: Host-related Performance of Cabbage Aphid 637

whether or not it was successfully established (i.e., was able to reproduce), age at first reproduc- tion (days), total number of nymphs laid during its life (2nd generation), duration of reproduction (days), and reproductive rate (number of nymphs laid per day). A rate of increase for each individ- ual aphid (ri) was calculated based on the aphid’s schedule of survivorship and fecundity:

where xi is age-class (days in this study), and Bxi is number of nymphs produced in that age-class (Lenski & Service 1982).

Establishment of a nymph on a given host was recorded as successful (1) if the nymph was able to reproduce, and unsuccessful (0) if it failed to re- produce. Because this response variable is binary, we analyzed the data using logistic regression (Quinn & Keough 2002). The model tests whether

βi = 0. The whole model is g (x) =

β0 (intercept) +

β1 (source host) +

β2 (recipient host) +

β3(source host _ recipient host), where g(x) is the natural log of the odds of establishment relative to non-estab- lishment. The model was tested by the log-likeli- hood test, G2 = -2[(log-likelihood reduced model)- (log-likelihood full model)] (i.e., the deviance). Re- duction in deviance attributed to the inclusion of predictor variables in the full model was tested for significance by a Chi-squared test. To test in- dividual coefficients, we used likelihood ratio tests by partitioning the deviance (Quinn & Ke- ough 2002). Analyses were performed with the JMP™ statistical package (v. 5.1.01.2, SAS Insti- tute 2003). In addition, the difference in the pro- portion of established versus non-established aphids within each recipient host was tested by the odds ratio analysis. The departure from zero was assessed by a G-test (Sokal & Rohlf 1995). The confidence interval was calculated for bino- mial proportions,

where p is the proportion of individuals estab- lished, q = 1 – p is the proportion of non-estab- lished individuals, and is the total of individuals assayed in each recipient host (Snedecor & Co- chran 1989).

For established females, age at first reproduc- tion, duration of reproduction, total number of nymphs, reproductive rate, and rate of individual increase (ri) were analyzed by means of two-factor ANOVA, with source and recipient host and inter- action as factors (Sokal & Rohlf 1995). A signifi- cant source

× recipient host interaction would in- dicate that the aphids’ performance changes be- tween the two environments (i.e., hosts). A signif-

icant effect of the source host may be indicative of genetic differences (or maternal effects) between the two aphid populations, whereas a significant recipient host effect may indicate differences in the general quality of host plants. Variables were log transformed (except ri) before performing the ANOVAs in order to approximate residuals to a normal distribution.

Phenotypic selection analyses were performed to determine whether different host species im- pose differential selection on aphids’ life history traits. The effect of both age at first reproduction and duration of reproduction on individual fitness was analyzed by means of multiple linear regres- sions within each recipient host (Lande & Arnold 1983). Regression analyses were performed with the standardized variables (i.e., mean = 0, stan- dard deviation = 1), and thus selection gradients are interpretable in terms of standard deviations (Lande & Arnold 1983). Covariance analysis was used to assess differences, if any, in the regression coefficients (slopes) between environments (host plants). Relative female fitness (wi) was estimated as the total number of nymphs laid divided by the mean number of nymphs in the population –w. Analyses were performed with the JMP™ statis- tical package (v. 5.1.01.2, SAS Institute 2003).

Performance and Natural Selection in the Presence of Maternal Effects

In the greenhouse, females randomly collected from each host in the field were allowed to pro- duce nymphs on the same species during three generations (acclimation period in the source host; Fig. 1). In each generation, nymphs were placed on a new single plant of the corresponding host species. Nymphs from the third generation were randomly selected and placed on individual plants of Br. oleraceae or Br. campestris (recipient host). One nymph was placed on an individual plant as previously described. For each individual aphid, we then recorded age at first reproduction, duration of reproduction, and number of offspring deposited until death. Using these data, we com- puted individual rate of increase (ri). Sample sizes of each source-recipient host combination were, as follows: Br. campestris to Br. campestris, 40; Br. campestris to Br. oleraceae, 34; Br. oleraceae to Br. campestris, 29; and Br. oleraceae to Br. oleraceae, 34. In order to assess the effect of source and re- cipient host on life history traits, a two-factor ANOVA was performed for each measured trait. A significant effect of the source host would indicate environmental effects upon life history traits, in- cluding maternal effects. On the contrary, a recip- ient effect would indicate that the host plant dis- tinctly affects aphids’ performance.

Finally, selection on life history traits (dura- tion of reproduction and age at first reproduction) was estimated for aphids derived from different

1 r i

x 1+( )–

x 0= ∑ Bxi=

p 1.96 pq n⁄+

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638 Florida Entomologist 92(4) December 2009

source hosts and developed on different recipient hosts, following the methodology stated above. Magnitude of selection between hosts was com- pared by means of an ANCOVA.

RESULTS

Performance and Natural Selection without Maternal Effects

The logistic regression model of establishment as a function of source, recipient host, and inter- action was statistically significant (-Log likeli- hood = 5.096, df = 3, χ2 = 10.192, P = 0.017, R2 = 0.089). Only the interaction source host _ recipi- ent host was significant (Table 1). Establishment

was higher when source host was similar to recip- ient host (Fig. 2a). The G-tests indicated signifi- cant differences in establishment between source hosts when the recipient host was Brassica olera- cea, while no difference was detected within Br. campestris (Fig. 2a).

Analyses of variance performed to assess the effect of the source and recipient host and interac- tion on life history characters indicated a signifi- cant effect of the recipient host on age at first re- production, total number of nymphs, reproductive rate, and individual rate of increase (ri), whereas no significant effect was detected on duration of reproduction (Table 2). Source host had an effect on individual fitness (i.e., number of nymphs), and source host × recipient host interaction did

Fig. 1. Schematic diagram of the reciprocal transfer experiment to estimate performance, natural selection, and maternal (source) host effects on life history traits of Brevicoryne brassicae growing on recipient hosts Brassica ol- eraceae and Brassicae campestris.

TABLE 1. LOGISTIC REGRESSION OF ESTABLISHMENT/NON ESTABLISHMENT OF BREVICORYNE BRASSICAE ON 2 HOST PLANTS (EXPERIMENT WITHOUT CONSIDERING MATERNAL EFFECTS). THE MODEL TESTS WHETHER β

I = 0. THE

WHOLE MODEL IS G (X) = β0 (INTERCEPT) + β1 (SOURCE HOST) + β2 (RECIPIENT HOST) + β3 (SOURCE HOST β2 RE- CIPIENT HOST), WHERE G (X) IS THE NATURAL LOG OF THE ODDS OF ESTABLISHMENT RELATIVE TO NON ES- TABLISHMENT. SAMPLE SIZE N = 86.

Parameter Estimate df χ2

(Likelihood ratio test) P

β0 (intercept) -0.618 1 6.170 0.013 β1 (Source host) -0.016 1 0.004 0.948 β2 (Recipient host) 0.330 1 1.821 0.177 β3 (source host × recipient host) -0.677 1 8.096 0.004

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Ruiz-Montoya & Núñez-Farfán: Host-related Performance of Cabbage Aphid 639

Fig. 2. Results of the experiment without considering maternal effects in the performance of Brevicoryne bras- sicae. (a) Average values of establishment (+ 1 confidence interval 95%), (b) age at first reproduction, (c) duration or reproduction, (d) average total number of nymphs, (e) reproductive rate, and (f) individual rate of increase [ri] measured in a reciprocal transplant experiment. From B to E +1SE is shown.

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640 Florida Entomologist 92(4) December 2009

not affect any variable (Table 2). On average, aphids reproduced earlier, at a higher rate, and produced more nymphs on Br. campestris than on Br. oleraceae (Figs. 2b, d-f). No significant effects of source and recipient host and interaction were detected on duration of reproduction (Table 2, Fig. 2c).

A positive relationship between duration of re- production and relative fitness was detected on both recipient hosts (Table 3 a-b). There were no differences in the selection gradients (i.e., equal slopes) between hosts for this character (AN- COVA, F = 2.211.469; df = 1,44, P = 0.14), indicat- ing that selection favored those individuals with a longer reproductive period regardless of their re- cipient host. No selection was detected upon the age at first reproduction in B. campestris (Table 3a). In contrast, relative fitness is related to age at first reproduction on Br. oleraceae (Table 3 a-b), indicating that phenotypes that began reproduc- tion earlier attained a higher relative fitness (Ta- ble 3a). Significant differences in the selection gradient on the age at first reproduction between recipient hosts were detected (ANCOVA, F = 4.46; df = 1,44; P = 0.04).

Natural Selection with Maternal Effects

Analyses of variance aimed to assess maternal effects upon life history traits of aphids indicated that the source host (on which the aphids spent 3

generations) did not explain a significant amount of variance (Table 4, Fig. 3a-d). However, a signif- icant effect of the recipient host was detected for the number of nymphs, ri, and marginally for age at first reproduction. The interaction source host x recipient host was not significant for all charac- ters (Table 4). As before, fecundity (number of nymphs and ri) was higher when aphids were grown on Br. campestris, irrespective of the source host (Fig. 3c-d).

Directional selection for duration of reproduc- tion was detected on both recipient hosts (Table 5) and of similar magnitude as indicated by the not significant recipient host x duration of reproduc- tion in an ANCOVA of fitness (F = 1.128; df = 1, 69; P = 0.29). No selection for the age at first re- production was detected in both hosts (Table 5).

DISCUSSION

The present study demonstrates that Br. campestris and Br. oleraceae constitute 2 different environments for B. brassicae. The probability of establishment of the aphids was higher when they colonized a host similar to their original host in the field. Also, fecundity and rate of increase was consistently greater for aphids reared on Br. campestris than on Br. oleraceae, indicating that the former host constitutes a higher quality habi- tat. Differences in performance displayed by B. brassicae between host plants cannot be attribut-

TABLE 2. ANALYSIS OF VARIANCE OF 4 LIFE HISTORY TRAITS OF BREVICORYNE BRASSICAE ON 2 HOST SPECIES (BRAS- SICA OLERACEAE AND BRASSICA CAMPESTRIS) IN A RECIPROCAL TRANSPLANT EXPERIMENT WITHOUT CONSID- ERING MATERNAL EFFECTS.

Variable Source of variation SS df F P

Age at 1st reproduction Source host 0.001 1 0.192 0.66 Recipient host 0.071 1 15.407 0.0003 Source × Recipient 0.003 1 0.591 0.44 Error 0.226 49

Number of nymphs Source host 0.188 1 3.925 0.05 Recipient host 1.109 1 23.117 <<0.001 Source × Recipient 0.051 1 1.070 0.31 Error 2.351 49

Duration of reproduction Source host 0.064 1 2.379 0.13 Recipient host 0.023 1 0.841 0.36 Source × Recipient 0.002 1 0.079 0.78 Error 1.321 49

Reproductive rate Source host 0.033 1 0.924 0.34 Recipient host 1.449 1 41.01 <<0.001 Source × Recipient 0.033 1 0.919 0.34 Error 1.731 49

Rate of increase (ri) Source host 1.48 × 10-3 1 0.637 0.43 Recipient host 0.107 1 45.93 <<0.001 Source × Recipient 3.84 × 10-3 1 1.65 0.20 Error 0.114 49

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Ruiz-Montoya & Núñez-Farfán: Host-related Performance of Cabbage Aphid 641

able to the environment previously experienced by the mother on the source host. Thus, aphid per- formance depends mainly upon the chosen or available host plant.

Variation of aphid performance on different hosts is frequently interpreted in terms of differ- ences in host plants phloem sap quality (Moran 1981; Sandström & Pettersson 1994; Dixon 1998).

Host quality can be defined as the plant’s chemical or physical attributes affecting aphid individual fit- ness (Awmack & Leather 2002). Brevicoryne campestris possesses a higher concentration of glu- cosinolates (Kjaer 1976) and leaf nitrogen than Br. oleraceae (Leal-Aguilar et al. 2008). These differ- ences between the 2 Brassica species may partly ex- plain observed differences in aphid performance.

TABLE 3. ANALYSIS OF SELECTION OF AGE AT FIRST REPRODUCTION AND DURATION OF BREVICORYNE BRASSICAE ON 2 HOST SPECIES (EXPERIMENT WITHOUT CONSIDERING MATERNAL EFFECTS). A. STANDARDIZED COEFFI- CIENTS (β) FROM LINEAR REGRESSION REPRESENT THE DIRECTIONAL SELECTION GRADIENTS. B. ANALYSIS OF VARIANCE AND ADJUSTED R2 FOR THE MULTIPLE REGRESSION MODEL.

-A-

Recipient host Character Coefficient β (SE) T P

Brassica campestris Intercept 1.000 (0.035) 27.93 <0.0001 Age first reproduction -0.005 (0.035) 0.13 0.83 Duration of reproduction 0.109 (0.036) 2.97 0.01

Brassica oleraceae Intercept 1.00 (0.077) 12.10 <0.0001 Age first reproduction -0.219 (0.080) -2.58 0.01 Duration of reproduction 0.245 (0.080) 2.87 <0.01

-B-

Recipient host Source SS MS Df F P r2 adjusted

Brassica campestris Model 0.306 0.153 2 4.42 0.023 0.21 Error 0.830 0.034 24 Total 1.137

Brassica oleraceae Model 3.148 1.574 2 10.12 <0.001 0.42 Error 3.578 0.156 23

Total 6.726

TABLE 4. ANALYSIS OF VARIANCE TO ASSESS EFFECTS OF THE MATERNAL HOST (SOURCE) AND RECIPIENT HOST ON LIFE HISTORY TRAITS OF BREVICORYNE BRASSICAE (EXPERIMENT IN THE PRESENCE OF MATERNAL EFFECTS).

Variable Source of variation Sums of Squares df F P

Age at 1st reproduction Source host 0.023 1 1.713 0.19 Recipient host 0.048 1 3.461 0.06 Source × Recipient 0.005 1 0.347 0.55 Error 0.956 69

Duration of reproduction Source host 0.009 1 0.111 0.74 Recipient host 0.019 1 0.242 0.62 Source × Recipient 0.015 1 0.196 0.65 Error 5.409 69

Number of nymphs Source host 0.025 1 0.143 0.71 Recipient host 0.810 1 4.572 0.03 Source × Recipient 0.024 1 0.138 0.71 Error 12.224 69

Rate of increase (ri) Source host 0.002 1 0.429 0.51 Recipient host 0.028 1 6.938 0.01 Source × Recipient 1.6 × 10-3 1 0.039 0.84 Error 69

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642 Florida Entomologist 92(4) December 2009

On Br. campestris, the reproduction period is tightly linked to number of nymphs (fitness), whereas on Br. oleraceae, duration of reproduc- tion and, indirectly, age at first reproduction are related to fitness. The magnitude of directional selection (β) on life history traits detected in this study (range 0.109 to 0.256) is higher than the av-

erage magnitude estimated for plants and ani- mals (0.08; Kingsolver et al. 2001), and is strong enough to promote both phenotypic differentia- tion and host specialization. However, the high gene flow between populations of B. brassicae as- sociated with both hosts may limit differentiation of host related populations (Ruiz-Montoya et al. 2003).

The differences found between hosts in estab- lishment, mean trait values, and selection on life history traits between recipients hosts (Table 3b) may suggest an initial stage of differentiation which could result in host race formation or host specialization in B. brassicae. The difference be- tween hosts in the probability of aphid establish- ment is consistent with the expectation of local adaptation to the native host. If success in estab- lishment is genetically correlated with fitness, further differentiation between host-associated populations may occur (Via 1991; Berlocher & Feder 2002; Drès & Mallet 2002). Our study offers a perspective on the role of selection and maternal effects in relation to host specialization in aphids. Yet, other potentially important factors in aphids like density dependence, predators, and parasi- toids could play a role and need to be addressed in future studies. For instance, host plant biased predation and parasitoidism upon B. brassicae may prevent or even counter specialization.

Evidence of host specialization has been docu- mented in the case of the aphid Acyrtosiphum pisum (Harris), (Homoptera: Aphididae) (Via 1991, 1999). In this species, it has been demon- strated that genetic correlations between re- source use and mate choice (habitat acceptance) is the most parsimonious explanation for ecologi- cal specialization of A. pisum to their hosts (al- falfa and clover; Hawthorne & Via 2001). Since B. brassicae reproduces only by parthenogenesis in the highlands of Chiapas, this might limit further divergence between host associated populations of B. brassicae.

Clonal reproduction of B. brassicae and a high probability for an aphid to encounter different po- tential hosts could promote the evolution of phe- notypic plasticity for resource use (Sultan & Spencer 2002). The evolution of host-specializa- tion depends upon the type of environmental vari- ation (fine- vs. coarse-grained; temporal vs. spa- tial), and the existence of genetic constraints (Via & Lande 1985; Fry 1996). Hence, various aspects of the B. brassicae-Brassica system warrant fur- ther study to test the Sultan and Spencer model (2002) in relation to the evolution of plasticity. First, at a fine scale, it is necessary to determine the existence of genetic constraints (trade-offs across host plants), reaction norms, and the ex- tent of gene flow among aphid populations (i.e., between hosts; Ruiz-Montoya et al. 2003), which hypothetically could counteract the effect of local selection (Sultan & Spencer 2002).

Fig. 3. Mean values of life history traits of Brevic- oryne brassicae aphids grown for 2 generations on a source (maternal) host, and grown and measured at the third generation on the recipient host (experi- ment of maternal effects). (a) Age at first reproduc- tion, (b) duration of reproduction, (c) average total number of nymphs, (d) rate of increase [ri]. Bars are mean +1SE.

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Ruiz-Montoya & Núñez-Farfán: Host-related Performance of Cabbage Aphid 643

ACKNOWLEDGMENTS

We are grateful to Manuel Giron Intzin for field and greenhouse assistance. C. A. Domínguez, L. E. Eguiarte, J. Fornoni, C. D. Schlichting, C. Cordero, M. Aluja, J. Sarquis, R. Dirzo, F. Vargas, P. Liedo, and A. F. G. Dixon made valuable suggestions to an earlier version of the manuscript. We thank Duncan Golicher for help in sta- tistical analysis. This research was supported by CON- ACyT (31543-B, 990039-DO) and ECOSUR (Population and Community Dynamics of Insects Laboratory) grants to L. R-M. This paper is a part of the Doctoral Dissertation of L. R-M in the Graduate Program in Bio- logical Sciences at UNAM.

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-A-

Recipient host Character Coefficient β (SE) t P

Brassica campestris Intercept 1.081 (0.096) 11.43 <0.0001 Age first reproduction -0.067 (0.097) -0.66 0.11 Duration of reproduction 0.343 (0.097) 3.52 0.001

Brassica oleraceae Intercept 1.000 (0.119) 8.39 <<0.001 Age first reproduction -0.202 (0.134) -1.51 0.14 Duration of reproduction 0.256 (0.134) 1.91 0.06

-B-

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Brassica campestris Model 5.331 2 6.718 0.003 0.21 Error 15.868 40 Total 21.199 42

Brassica oleraceae Model 4.371 2 5.126 0.01 0.22 Error 11.512 27 Total 15.883 29

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