Caste Asymmetries in the Neotropical Swarm- Founding Wasp

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Caste Asymmetries in the Neotropical

Swarm-Founding Wasp

Polybia (Trychothorax) ignobilis

(Hymenoptera, Vespidae, Epiponini)

by

Ivan Cesar Desuó1_{; Sulene Noriko Shima}1_{; Alexandre Capelete Lima Oliveira}2_; Bruno Gomes2_{& Carlos Tadeu Santos Dias}3

ABSTrACT

Epiponini wasps are dominant in Neotropical regions and represent, in

several ways, exceptions for general theories regarding social insects. They are

highly eusocial and most species build large and complex nests. These wasps

form large populations, and colony reproductive fission occurs by swarming.

On the other hand, differently from other highly social insects, in several species

morphological differences among castes are slight or absent, suggesting that

caste determination in the Epiponini differs from better known systems. In

fact, the mechanisms that lead to caste differentiation in this tribe still remain

obscure. The aim of this study is to investigate the pattern of caste

differentia-tion during the colonial cycle of the Neotropical Epiponini wasp

Polybia (T.)

ignobilis

. Measurements were taken from 13 morphometric variables regarding

the head, metassoma, messosoma, and wing. Ovarian development, relative

age, and fat bodies were also examined. Four patterns of ovarian development

were found and varied from filamentous ovarioles with no visible oocytes to

well-developeded ovaries with mature oocytes. ANOVA showed that queens

were significantly larger than workers in all colony phases, and also showed

some significant differences between workers and intermediates in some

body parts. However, multivariate statistics revealed that intermediates are

in fact morphologically similar to workers, whereas queens were identified as

a different group composed of larger individuals. Caste differentiation varied

according to the colony cycle: the morphological differences between queens

and workers tend to increase with colony maturity.

1_{Departamento de Zoologia, Instituto de Biociências, UNESP, Campus rio Claro, SP, Brazil, ivan.} desuo@yahoo.com.br, CEP:13506-900, (5519) 35264298.

2_{Departamento de Zoologia e Botânica, IBILCE, UNESP, Campus São José do rio Preto, SP, Brazil,} CEP:15054-000, (5519)32212373.

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52 Sociobiology Vol. 57, No. 1, 2011

Key-Words:

Social Insects, caste differences, kin selection, cyclical

oli-gogyny, Vespidae, Epiponini.

INTrODUCTION

The division into castes is a keystone in studies of social evolution in

social insects, and a higher degree of caste differentiation usually indicates a

well-defined division of labor and, consequently, a higher level of sociality

(Bourke 1999; Noll

et al.

2004). In this context, the Vespidae attracts special

attention of sociobiologists, because a remarkable sequence of sociality

pat-terns is found in this group, and every step of social organization is observed

(Wilson 1971).

The Neotropical Epiponini comprises 19 genera and at least 229

spe-cies representing the Neotropical swarm-founding wasps, distributed from

Argentina to the southwest of the USA (Carpenter 2004). In Brazil the

Epiponini are ecologically dominant, showing the highest level of diversity

and endemism ( Jeanne 1975 1980). The Epiponini present polygyny and a

complex caste system (Carpenter 1993, 1997), and represents an important

group in caste evolutionary studies in social wasps (richards & richards

1971; richards 1971; richards 1978). Unlike other highly eusocial insects

(such as honeybees and vespines), societies of the Epiponini are better

char-acterized as a “conspiracy of workers” and, apparently, queens do not play

an important role in the control of sociality (Noll & Wenzel 2008). The

Epiponini are also polygynous and queen demography is regulated by worker

policing, which usually selects only the queens with the highest

reproduc-tive status (West-Eberhard 1978). This flexibility in the number of queens

results in few queens in the advanced phases of the colony cycle and allows

the maintenance of a reasonable level of genetic relatedness among nestmates,

representing a remarkable manifestation of kin selection theory (Hamilton

1964a,b; Hamilton 1972). Furthermore, the presence of intermediate females

brings more complexity to caste determination as these females may show

totipotency (e.g.,

Parachartergus colobopterus

, Strassmann

et al.

2002), may

lay eggs, and also seem to play an important role in the process of cyclical

oligogyny, which regulates the number of queens in the colony (Strassmann

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richards (1978) documented that castes in Polistinae (mainly in the

Epiponini) present three main types of differentiation: (1) queens larger than

workers [e.g.

A. areata

(Jeanne & Fagen 1974);

A. vicina

(Sakagami

et al.

1996);

Protonectarina sylveirae

(Shima

et al.

1996b);

Epipona guerini

(Hunt

et al.

1996);

Polybia scutellaris

(Noll

et al.

1997a)]; (2) conspicuous dimorphism,

in which queens present different body proportions compared to workers,

resulting in shape differences. This could be a consequence of differential

ontogenetic reprogramming of growth parameters ( Jeanne

et al.

1995), and

in this case intermediate females are absent [e.g.

Apoica flavissima

(Shima

et

al.

1994);

Polybia dimidiata

(Shima

et al.

1996a)]; (3) slight or undetectable

differences with the presence of intermediate females [e.g.

Pseudopolybia

vespiceps

(Shima

et al.

1998);

Protopolybia exigua

(Noll

et al.

1996)].

Studies on castes of Epiponini wasps through the colony cycle have

in-creased considerably during the last decade. According to Noll and Zucchi

(2002), five different types of social regulation are now recognized within

this tribe: (1) absence of morphological differentiation between queens and

workers during the whole nest cycle, in which the intermediate females may

achieve reproductive status [e.g.,

Chartergellus communis

(Mateus 1999);

Parachartergus smithii

(Mateus 1997);

Pseudopolybia vespiceps

(Shima

et al.

1998

); Brachygastra lecheguana

(Shima

et al.

2000)]; (2) absence of caste

differences, in which young females may develop ovaries only during some

phases of the colony cycle [e.g.,

Metapolybia aztecoides

and

Synoeca.

suri-namensis

(West-Eberhard 1981),

Synoeca cyanea

(Noda

et al.

2003)]; (3)

morphological discontinuities among castes varying according to the nest

cycle and the constant presence of intermediate females [e.g.

Protopolybia

exígua

and

P. sedula

(Noll & Zucchi 2002)]; (4) presence of uninseminated

egg-laying females only during some phases; the differences between castes

increase throughout the colony cycle [e.g.

Chartergus globiventris

(Noll &

Zucchi 2002);

Polybia scutellaris, P. paulista

,

and

P. occidentalis

(Noll &

Zuc-chi 2002)]; (5) morphological differences between queens and workers are

conspicuous during the whole colony cycle and there are no uninseminated

egg-laying females [e.g.

Apoica flavissima

(Shima

et al.

1994)

; A. pallens

(4)

al.

(2008) these syndromes are easily interpreted phylogenetically and suggest

that castes in Epiponini evolved several times in different lineages, however

from an ancient condition of casteless societies.

The Neotropical Epiponini wasp

Polybia (Trychothorax) ignobilis

(Haliday,

1836) is widely distributed throughout South America, occurring from Panama

to Paraguay and Argentina. This species is extremely aggressive and builds

enveloped nests inside cavities (richards 1978). The aim of this study was to

investigate the patterns of caste differentiation in

P. (T.) ignobilis

during the

colony cycle and address questions regarding the role of cyclical oligogyny

in the caste system of this species.

Material and Methods

For this study, six colonies of

Polybia (Trichothorax) ignobilis

in different

phases of development were analyzed. All colonies were collected in the

state of São Paulo, Brazil, in the following cities: colonies I (30/04/95), II

(20/11/95), III (31/07/96), and V (28/09/1995) were collected in rio Claro,

(22º24’36’’S; 47º33’36’’W); colonies IV (30/03/2007) and VI (22/10/93)

in ribeirão Preto (21º12’’42’S; 47º48’24’’W). All adult wasps from the

colonies were immediately killed and fixed in Dietrich’s solution and then

preserved in 70% ethanol.

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In order to assess the reproductive status of each female picked for the

morphological study, they were dissected under a stereomicroscope and their

ovaries were drawn, permitting the establishment of an ovarian development

pattern. Inseminated females with well-developeded ovaries were regarded as

queens. Unmated females with no or a low degree of ovary development were

regarded as workers. When the females were not inseminated but presented

substantial ovarian development, they were regarded as intermediates. The

relative age was estimated by observing the progressive pigmentation of the

transverse apodeme across the hidden base of the 4

th

_{sternum. According to}

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richard & richards (1951), richards (1971), and West-Eberhard (1973), the

progressive pigmentation of this structure is related to the age of individuals.

Females were classified as: (1) without pigmentation, (2) light brown, (3)

dark brown, and (4) black.

We used SAS 9.2 for Windows to perform statistical analysis. All

measure-ments herein given in millimeters were log converted to facilitate arranging

them in a normal distribution. From the measurements, the means and

standard deviations were calculated. Aiming to verify the differences among

queens, workers and intermediates, a One-Way ANOVA was performed

using PrOC ANOVA procedure using the option BON to test the means

differences using the Bonferroni-correction

t

-test.

Discriminant analysis was conduct using PrOC DISCrIM procedure

in SAS to allocate the intermediates females into worker or queen classes.

We used the option POOL=TEST to test homogeneity of covariance

matrices within groups, and if the null hypothesis was rejected, then the

software proceeded the quadratic form of discriminant function. We used

the CrOSSVALIDATE option to provide rigorous classification accuracy

and the misclassified observation was listed using CrOSSLISTEr option

in SAS software.

Canonical variate analysis (CVA) was implemented to describe differences

among queens, workers and intermediates. This procedure finds a new set

of axes or canonical variates that are linear combinations of the original set

of variables, which provide the best discrimination between groups by

maxi-mizing the between-group variance relative to the within-group variance .

To perform this analysis we used PrOC CANDISC in SAS software. The

Canonical Plots were obtained using JMP 7.0 linked to SAS database.

Finally, we used PrOC STEPDISC to perform a stepwise discriminant

analysis in order to choose the most important variables which discriminate

castes in

P. (T.) sericea

. We use the option SLENTrY=0.15, which specifies

the significance level for adding variables, in this case, variables with a P-value

greater than 0.15 were removed from the model. Graphs were made using

JMP 7.0 Statistical Software linked to SAS 9.2 database.

rESULTS

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relative age of individuals, nest architecture,

and presence of males (Table 1). Based on this

analysis and according to Noll & Zucchi (2000,

2002), the colonies were classified as following:

(Colony I) - Swarm; (Colonies II and III) - Pre

emergence, (Colonies IV, V and VII) - Male

Production.

The results regarding the ovarian

develop-ment of

Polybia (T.) ignobilis

are shown in Fig.

2. Four general patterns of ovarian development

were found, although some variation was

ob-served: pattern A - filamentous ovarioles with

no visible oocytes; pattern B: B1 - ovaries with

slight oocyte development; pattern B2 -

ovari-oles with small well defined oocytes; pattern C:

C1 - characterized by only one nearly mature

oocyte located in the base of the ovary; pattern

C2 – more than one nearly mature oocyte in

the basal region of the ovary; pattern D: D1 –

well-developed ovaries with mature oocytes;

pattern D2 – extremely well-developed (and

usually twisted) ovaries with many nearly

ma-ture oocytes in each ovariole.

Females that were not inseminated and

pre-sented patterns A, B1, and B2 were regarded

as workers; those which presented patterns C1

and C2 were regarded as intermediates. Since

insemination was exclusively detected in females

presenting type D1 and D2 ovaries, these were

considered queens.

The data on relative age (Fig. 3) showed that

workers of colony I were represented mostly

by young individuals (Patterns 1 and 2), in the

other five colonies workers were characterized

by different-aged broods. Queens were always

T ab le 1 . Co lo ny a nd N es t c om posit io n (

W – W

or

ke

rs, I – I

nt

er

me

dia

tes, Q – Q

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the oldest individuals (Patterns 3 and 4), and young queens were found only

in colonies II, II and IV in small percentages. Intermediate females were

represented mostly by young individuals.

Univariate analysis of variance (ANOVA) showed that queens were

sig-nificantly larger than workers and intermediates for most characters; this

pattern occurred in all colonies (Table 2). Some differences between workers

and intermediates were found in colonies II, III, IV and V. Except for colony

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IV; intermediates were smaller than workers in some variables (Table 2). The

results of MANOVA confirmed that queens were larger than workers and

intermediates, as multivariate contrasts were strongly significant (Table 3).

Considering all variables together intermediates differed from workers only

in colonies IV and V (Table 4).

Discriminant function analysis allocated the intermediates mostly within

the worker group and the percentage of correctly classified intermediates

was noticeablely low. Otherwise, queens presented a higher percentage of

correct classification, which reinforces the results cited above (Table 3). The

canonical Variate plot (Fig. 4) clearly shows that queens form a well

sepa-rated group from workers and intermediates in every phase of the colonial

cycle. However, such a distinction was not observed between workers and

intermediates. Queens are better characterized for higher values of CAN1,

which is mostly represented by abdominal variables, such as T4H and T4W

(see the values of CAN1 in Table 2). Thus, queens are represented by larger

individuals, especially in the abdominal characters. However, a contrast among

abdominal variables, head length and wing length may also be important in

discriminating castes in this species (Table 2)

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60

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cio

bio

lo

gy V

ol. 57, N

o. 1, 2011

Table 2. Mean and standard deviationof most important variables, One-Way ANOVA significance, Stepwise Discriminant Analysis results and Canonical Scores of Polybia (T.) ignobilis. Q-queens, W-Workers, I-Intermediates.

Colony Characters Q WMean±SD I ANOVA F λ F to Enter p>F Can1 Can2

Colony I

T4H 1.78±0.11 1.49±0.05 - 239.89d 0.32 239.89 <0.001 1.12

-T4W 4.90±0.15 4.37±0.30 - 10.62d 0.28 14.81 <0.001 0.64

-MSW 2.29±0.07 2.13±0.22 - 9.63d 0.27 4.48 0.0366 0.47

-HW 3.03±0.03 3.00±0.06 - 2.07ns 0.25 9.19 0.0030 -0.35

-Colony II

T4H 1.83±0.07 1.53±0.08 1.49±0.05 380.44a,b 0.19 380.44 <0.001 1.98 -0.46

MSL 2.49±0.10 2.45±0.14 2.34±0.09 8.99a,c 0.14 34.62 <0.001 0.79 0.81

MSW 2.47±0.08 2.27±0.05 2.24±0.04 173.41a,b 0.12 11.13 <0.001 0.83 -0.11

WL 4.54±0.12 4.46±0.15 4.37±0.09 11.64a,b 0.11 7.56 <0.001 -0.38 0.52

T4W 5.25±0.37 4.69±0.19 4.63±0.12 260.14a,b 0.11 5.30 0.006 0.75 -0.21

HW 3.16±0.06 3.09±0.08 3.09±0.08 18.61a,b 0.10 2.62 0.075 -0.19 -0.43

Colony III

T4H 1.81±0.07 1.53±0.09 1.50±0.05 525.96a,b 0.15 525.96 <0.001 2.10 1.09

HW 3.02±0.04 3.03±0.05 2.98±0.04 3.68c 0.11 33.32 <0.001 -0.78 -0.55

T4W 5.06±0.12 4.57±0.14 4.47±0.10 328.26a,b 0.09 20.16 <0.001 0.82 -0.54

T1AH 0.99±0.06 0.87±0.04 0.86±0.05 124.89a,b 0.08 4.01 0.02 0.31 0.07

AL 4.79±0.09 4.56±0.14 4.50±0.08 88.35a,b 0.08 2.85 0.06 0.29 0.33

Colony IV

T4H 1.77±0.04 1.52±0.06 1.57±0.04 142.71a,c 0.30 142.71 <0.001 1.45 -1.41

T1AH 1.06±0.06 0.92±0.05 0.94±0.05 59.83a,b 0.26 8.96 <0.001 0.44 -0.55

T4W 4.95±0.13 4.52±0.14 4.75±0.08 79.81a,c 0.23 7.37 0.001 0.24 1.70

HW 3.07±0.04 3.05±0.06 3.06±0.03 0.63ns 0.20 8.84 <0.001 -0.42 -0.38

MSW 2.40±0.08 2.19±0.06 2.26±0.07 94.16a,c 0.19 3.48 0.03 0.62 0.15

AL 4.94±0.16 4.71±0.26 4.85±0.09 16.58d 0.18 2.88 0.06 0.29 0.33

a – Queens larger than workers and intermediates (Bonferroni p<0.05); b – No differences between workers and intermediates (Bonferroni p<0.05); c – Workers and Intermediates differ significantly (Bonferroni P<0.05); d – Queens larger than workers (Bonferroni p<0.05);

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61

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Table 2. Mean and standard deviationof most important variables, One-Way ANOVA significance, Stepwise Discriminant Analysis results and Canonical Scores of Polybia (T.) ignobilis. (Continued). Q-queens, W-Workers, I-Intermediates.

Colony Characters Mean±SDQ W I ANOVA F λ F to Enter p>F Can1 Can2

Colony V

T4H 1.78±0.04 1.50±0.06 1.52±0.32 128.07a,b 0.45 128.07 <0.001 1.29 -0.64

HW 3.11±0.04 3.09±0.07 3.03±0.08 14.60c,e 0.36 26.49 <0.001 -0.73 0.40

MSW 2.51±0.06 2.25±0.08 2.18±0.09 82.52a,c 0.29 20.88 <0.001 0.72 0.62

WL 4.51±0.09 4.43±0.15 4.27±0.30 17.30a,c 0.27 9.13 <0.001 -0.44 0.70

T1AH 1.03±0.04 0.88±0.05 0.87±0.05 51.50a,b 0.25 5.88 0.003 0.29 -0.39

T4W 5.07±0.12 4.54±0.15 4.43±0.29 99.35a,c 0.24 4.21 0.02 0.42 0.15

Colony VI

T4W 4.87±0.23 4.50±0.16 4.50±0.11 55.94a,b 0.57 55.94 <0.001 0.98 0.31

HW 3.01±0.08 3.02±0.06 3.00±0.07 0.03ns 0.45 18.74 <0.001 -0.71 -0.17

T4H 1.61±0.10 1.48±0.06 1.44±0.06 40.02a,b 0.40 10.28 <0.001 0.78 -1.11

MSL 2.33±0.11 2.31±0.10 2.31±0.08 0.51ns 0.38 3.13 0.05 0.41 -0.48

MSW 2.30±0.10 2.16±0.07 2.16±0.08 32.97a,b 0.35 6.38 0.002 0.63 -0.002

T1AH 0.94±0.07 0.88±0.06 0.90±0.05 11.54a,b 0.34 1.99 0.14 0.10 0.94

a – Queens larger than workers and intermediates (Bonferroni p<0.05); b – No differences between workers and intermediates (Bonferroni p<0.05); c – Workers and Intermediates differ significantly (Bonferroni P<0.05); d – Queens larger than workers (Bonferroni p<0.05);

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The values of Wilks’ lambda (λ) are shown in Table 2. The mean values of λ

ranged from 0.1 to 0.5, indicating that the characters are good discriminators

and castes in this species are well defined.

Table 3. Classification matrix for group comparison after discriminant function analysis, using the option CrOSSVALISATE in SAS software.

Correctly Classified Females (%)

Predicted Classification Workers Intermediates Queens

Colony I WorkersQueens 100.00100.00 1000 -- 09

Total 100 100 0 9

Colony II

Workers 72.00 72 27 1

Intermediates 1.37 12 1 0

Queens 98.66 0 1 72

Total 59.44 84 29 73

Colony III WorkersIntermediates 89.0010.00 899 111 00

Queens 100.00 0 0 76

Total 66.33 98 12 76

Colony IV

Workers 96.94 95 1 2

Intermediates 0.00 8 0 1

Queens 94,74 1 0 18

Total 63.89 104 1 21

Colony V WorkersIntermediates 66,0068,00 6632 3468 00

Queens 100,00 0 0 12

Total 78.00 98 102 12

Colony VI WorkersIntermediates 66,0041.67 6613 2810 61

Queens 77.78 4 2 21

Total 28.00 28 40 20

Table 4. results of MANOVA. Multivariate contrasts considering all variables together (Q-queens, W-Workers, I-Intermediates).

Colony _F W vs. Q _p Multivariate Contrast_F W vs. I_p _F I vs. Q _p Colony I

Wilk´s Lambda 40.02 <0.0001 - - 20.52 <0.0001 Colony II

Wilk´s Lambda 164.89 <0.0001 1.81 0.0774 44.72 <0.0001 Colony III

Wilk´s Lambda 225.35 <0.0001 1.74 0.0918 49.77 <0.0001 Colony IV

Wilk´s Lambda 55.23 <0.0001 3.39 <0.001 14.43 <0.0001 Colony V

Wilk´s Lambda 59.80 <0.0001 4.09 <0.001 59.97 <0.0001 Colony VI

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DISCUSSION

In

Polybia (T.) ignobilis

, queens were physiologically distinct from workers

and intermediates. They presented more developed ovaries, a larger fat body,

they were the oldest individuals and were the only inseminated females, a

pattern also observed in several species of Epiponini (Noll

et al.

2004; Noll

& Zucchi 2000; Noll

et al.

1997a,b; Shima

et al.

1996a,b; Shima

et al.

1994)

(Fig. 2 and 3). However, some flexibility in the ovarian development of queens

was observed: in the early stages of colonial cycle queens presented smaller

ovaries with fewer eggs and oocytes in the final stage of development than

in advanced stages (see Fig. 2). This is evidence of the actuation of worker

policing and confirms cyclical oligogyny and queen selection for this species.

This mechanism selects only the most productive queens in the colony (those

with larger ovaries and fat bodies, and longer longevity), and the maintenance

of queen dominance may inhibit ovarian development in workers. In fact,

Strassmann

et al.

(2002) verified that, in manipulated colonies of

Parachartergus

colobopterus

from which queens were removed, 86.8% of workers developed

their ovaries. As some species may present totipotency (see Strassmann

et al.

2002 and West-Eberhard 1978 and 1981) the loss of queen dominance may

represent a chance for workers to remain longer in the nest, feeding more,

accumulating a greater fat body and becoming reproductive.

The role of intermediate females in the Epiponini has been gradually

de-fined (Desuó

et al. 2

009). Some authors suggested that intermediates could

represent young or noninseminated queens (Forsyth 1978; West-Eberhard

1978; and Strassmann

et al.

2002). Naumann (1970) proposed that these

fe-males are important for the energetic flow within the colony, since their main

function would be laying trophy eggs. Intermediates could also be responsible

for producing males, as stated by Simões (1997). However, ratnieks (1988)

demonstrated that when queen mating frequency exceeds two, worker

polic-ing acts against the production of males by other workers or intermediates.

Moreover, the authors (1998) showed that males of

Brachygastra mellifica

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2004). This process is likely to be related to queen senescence, which probably

leads to a decrease in the efficiency of pheromones and loss of reproductive

dominance by queens, although the mechanisms related to this process are not

clear yet. Nascimento

et al.

(2004) showed that cyclical oligogyny in

Asteloeca

(15)

ujhelyii is behaviorally mediated, and the authors observed the occurrence

of conflicts among queens and workers/queens; however, the first seems to

be more related to the initial phases of colony development. Nevertheless,

few studies addressed the role of intermediates in queen elimination in

Epi-ponini. Mateus (2005) analyzed the behavioral repertoire of intermediates

of

Parachartergus fraternus

and showed that these females also acted directly

in queen elimination through aggressive acts. In this study, intermediates

competed against queens for reproductive dominance through differential

oophagy. Morphological and physiological data showed that intermediates

of

Polybia (T.) ignobilis

are young workers, with more fat body and more

developed ovaries.

Several syndromes of caste differentiation are found in

Polybia

genera

(Noll

et al.

2004; Noll and Wenzel 2008). This study showed that castes in

Polybia (T.) ignobilis

are characterized mainly by differences in overall size

of females (Table 2). Queens were larger than workers and intermediates,

especially in the abdominal variables, which presented the higher values of

CAN1 (Table 2). However, multivariate analysis also revealed that a contrast

among variables may discriminate castes in

P. (T.) ignobilis

, meaning that a

shape component may be important in caste differentiation in this species.

Table 2 shows this contrast and indicates that individuals which present

larger abdomens also present smaller heads and wing length. Such a pattern

were reported for other species of

Polybia

and it is a widespread pattern of

caste differentiation in the Epiponini (Desuó

et al.

2009; Noll

et al.

2004).

According to Jeanne

et al.

(1995) such differences are related to a differential

growth of imaginal discs during immature period, however, developmental

studies are need to evaluate this theory.

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1978) and other

Polybia

species (Noll and Zucchi 2000), and in the present

study for

Polybia (T.) ignobilis

(Fig. 3). The canonical Scatterplot (Fig. 4)

indicates that caste differences increased as the colony developed and castes

are not well defined during initial phases of colonial cycle (Colony I - swarm)

and final stages (Colony VI - late male production).

Such differences through the colony cycle may indicate that body size may

be important in competition among queens, since larger queens tend to be

more reproductively dominant and a hierarchical system based on

repro-ductive dominance should select larger individuals during variations in the

number of queens through the colonial cycle (Queller & Strassamann 1989;

reeve 1991; Noll & Zucchi 2000). Another important implication of queen

selection regards the maintenance of reasonable levels of genetic relatedness

in the advanced phases of colonial cycle, making Hamiltons’ kin selection

applicable (Nascimento

et al.

2004). Because of the importance of workers

in the control of queen demography, Noll and Wenzel (2008) defined the

Epiponini society as “a conspiracy of workers”.

According to Bourke (1999) and Strassmann

et al.

(2002), colony size

may also influence caste differentiation. In social wasps which present small

colonies (i.e.,

Polistes

,

Mischocyttarus

) no morphological caste differences are

observed, and then small societies are better characterized by direct conflicts

for reproduction. On the other hand, in insects which build large colonies

(i.e., ants, termites, honeybees, stingless bees, and vespines) and also present

monogyny, castes are remarkable and the conflicts for reproduction tend to

be indirect, reflecting directly in kin composition. Once again the Epiponini

represent an important exception for this generalization, since in several

spe-cies which build large colonies, morphological castes are absent or slightly

defined (richards 1978; Jeanne 1980; Jeanne

et al. 1

995; Hunt

et al.

1996;

O’Donnell 1998; Strassmann

et al.

2002; Noll & Wenzel 2008). According to

Strassmann

et al.

(2002), the low level of caste differentiation in the Epiponini

is the result of a post-imaginal (no differential feed during larval period) caste

determination. For these authors, females of some species are totipotent at

emergence as adults, but they are quickly suppressed by other workers when

more than one queen is present. This view matches that of West-Eberhard

(1978 and 1981) in

Metapolybia aztecoides

and

Synoeca surinama

. Mateus

(17)

post-imaginally determined. However, based on physiological and morphological

data presented in this study caste determination in

Polybia (T.) ignobilis

has

a strong pre-imaginal component, a result of differential feeding during the

larval stage or developmental switches responsible for the shape differences

detected ( Jeanne

et al.

1995) .

ACkNOWLEDGMENTS

The authors gratefully acknowledge the financial support of CNPq. We

also give special thanks to Prof. Dr. Sidnei Mateus and Prof. Dr. ronaldo

Zucchi for their suggestions and for providing colony VIII, and to Mr. Jaime

r. Somera, the illustration expert, for the artwork.

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