Table Of Content1992. TheJournal ofArachnology 20:200-206
SEX RATIO IN THE SOCIAL SPIDER DIAEA SOCIALIS
(ARANEAE: THOMISIDAE)
David M. Rowell: Division ofBotany and Zoology, Australian National University,
GPO Box 4, Canberra City 2601 Australia
Barbara York Main: Zoology Department, University ofWestern Australia,
WA
Nedlands, 6009 Australia
ABSTRACT. SexratiodatafromembryosandadultsarecomparedinthesocialthomisidDiaeasocialis.The
relative proportions ofmales and females do not differ significantly between the two data sets, indicatingthat
a sex ratio bias already exists at the time offertilization. A statistical comparison with published data for the
theridiidAnelosimuseximiusshows a different ratio but similar pattern forthetwo types ofdata. A molecular
procedurefordeterminingwhethertheoverproductionoffemalesresultsfromabiasinthespermordifferential
success ofsperm in fertilization is described.
Inthosesocialspiderswheretheadultsexratio ization, and preferably before hatching. Using
isknown,itisfemale-biased(Buskirk 1981;Voll- cytological techniques, Aviles and Maddison
rath 1986). However, Fisher’s principle (Fisher (1991)havedemonstratedaprimaryfemalebias
1930) predicts that selection will favor an equal in embryos ofthe social species Anelosimus ex-
parentalinvestmentin offspringofboth sexes. It imius Keyserling andA. domingo Levi (propor-
would follow, then, that any bias in sex ratio tionofmales=0.08and0.09,respectively)which
should result from forces acting after dispersal. contrasts with a more even sex ratio for non-
On the basisofthecorrelationbetween sexratio social species in the genus.
and social behavior it would seem reasonable to In spidersit isrelativelyeasytodeterminethe
hypothesizethatthetwophenomenaareinsome gender ofembryos using cytological techniques
waycausallylinked. Skewedsexratioshavesim- because of their unusual sex determination
ilarly been reported in coloniesofDiaeasocialis mechanism. In 81% of the spider species that
Main, with an observed male:female ratio of have been analyzed there are two sex chromo-
0.2126 (Main 1988). somes involved in sex-determination (White
The life-history and behavior of D. socialis 1973). Males possess one copy ofeach ofthese,
were described in Main (1988). Colonies are while females cany' two copies ofeach (four in
foundedby individualgravid females which use all). Consequently, femaleshavetwo morechro-
silk to bind together eucalypt leaves to form a mosomes than males, and so can be distin-
nest. The brood spiderlings remain with the guished simply on the basis of chromosome
mother after hatching, add leaves cooperatively number. This method of sex-determination is
toenlargethenest, shareprey,matureinthenest referred to as XjXjO, and it would appear to be
and mate with siblings. Generally, only mated the ancestral condition in spiders (White 1973).
females migrate from the nest, overthe Septem- This system has been modified to X.XjXjO in
bertoNovemberperiod. Thesexratiosreported the Australian huntsman spiders (Rowell 1985)
ofonratdhuilstsapnecdiperse(adMualitnmo19r8p8h)olhoagvyeobneleyn,aanssdecsosne-d akinmdeXns1iXs2X(D3aXtt4Oai&ntChheatstpearrjaesesi1d9H83e)t.ewInpotdheamsiak--
sequently the observed bias may reflect differ- jorityoftheOxyopidaeanalyzed, therehasbeen
ential life span or mortality between the sexes. areductioninsexchromosomenumberresulting
XO
Alternatively, it is possible that the sex ratio in an system similarto that prevalent in the
is skewed from birth as a result ofmeiotic drive insects (Datta & Chatterjee 1983). Thus, among
orsomeotherfactor.Todeterminewhethersuch different species, females may possess one, two,
processes are operating requires the ascertain- three or four more chromosomes than males.
ment ofgender as soon as possible after fertil- Over300speciesofspiderhavebeenanalyzed
200
ROWELL& MAIN-SEX RATIO IN A SOCIAL SPIDER 201
simuseximius,todeterminewhetheraparticular
primary sex ratio may exist in social spiders.
METHODS
All specimens, both eggs and males, used in
thecytological analyses were collected by one of
us (BYM) from sites in the vicinity ofTorbay
westofAlbany, southwesternWesternAustralia.
The sex ratio was also determined from adult
and subadult spiders from colonies collected in
the same areas by BYM (subsequently consid-
eredbyMain(1988))andfrom PembertonbyT.
A.Evans.Inyoungercoloniesattwositesasmall
proportion ofjuvenile individuals possessed no
characters ofeither sex. No reliable morpholog-
ical charactershavebeen found forsexingyoun-
gerspiders,andthesmallsizeofD. socialismakes
dissection of large numbers of individuals im-
Figure 1.—Male meiosis in D. socialis (2N = 24). practical. However, the authors’ previous expe-
Note the presence ofeleven bivalents plus two heter- rience with this species has shown that the rate
omorphic X-chromosomes (arrow). ofdevelopmentwithin and between the sexes in
a colony is concerted (but with males maturing
cytologically, and the kind ofsex-determination beforefemales).Henceswellingofthepalpwould
system describedaboveisvirtuallyuniversal. In havebeenapparentweretheseindividualsmale.
only two cases, the sparassid Delena cancehdes Consequently, in colonies possessing a mixture
WalckenaerandthesalticidgenusPellenesSimon, ofadultsandjuveniles,thejuvenileswerescored
do aberrant systems exist (Rowell 1990; Mad- as females. The validity ofthis assumption was
dison 1982), both involving rearrangements be- borne outin thestatisticalanalysis, whereitwas
tweentheX-chromosomesandautosomes.These shown that the sex ratios within these colonies
modified systems are very distinctive, however, werenotstatisticallydifferentfromthoseofadult
and can easily be identified from male meiotic colonies. Colonies in which the majority ofin-
preparations. dividualswerejuvenilewere not included in the
In this study, sex ratios ofadult and subadult analysis, nor were degenerate colonies with less
D. socialisarecomparedtoembryonicsexratios than five individuals, orthosewithout represen-
determined by cytological means, to ascertain tatives ofboth sexes (two colonies qualified for
whethertheobservedbiasarisesbeforehatching, the latter).
orasaresultofdifferentialmortalityofthesexes. Chromosome preparations were made from
In addition, the ratios observed are compared testes and embryos using the techniques de-
with those reported by Aviles and Maddison scribed in Rowell (1985) which differs from that
(1991)fromcytologicalanalysisandAviles(1986) detailed by Aviles and Maddison (1991). Not
from morphology for the social species Anelo- every embryo yielded spreads which could be
Table 1.—Heterogeneityand deviation from even sex ratio in embryonicdata forD. socialis. *P < 0.01.
Male
Site Males Females Total proportion
Coombes Rd, Torbay 5 13 18 0.28
Coombes/Harding Rds, Torbay 3 8 11 0.27
Coombes/HardingRds, Torbay 4 12 16 0.25
Coombes/Harding Rds, Torbay 4 9 13 0.31
Total 16 42 58 0.28
Deviation from 1:1 G = 12.08* HeterogeneityG = 0.122 ns.
202 THEJOURNALOFARACHNOLOGY
2
)> cue
fO
!!
M u
H<t
ta ••
^
X, X
1 2
H
K n
I*
••
la li II II
• •
Figures2, 3.—KaryotypesfromD.socialisembryos:2,male(2N =24);3,female(2N=26).Notethepresence
ofsupernumerary microchromosomes in the two karyotypes.
counted with certainty, but given the marked The data obtained for D. socialis were com-
developmental synchrony within each egg sac, pared, by G-test, against the datagiven forAne-
those counted are held to represent a random losimuseximiusbyAvilesand Maddison (1991)
sample from each colony. and by Aviles (1986).
Data were analyzed for deviation from a sex
RESULTS
ratio of 1:1 and homogeneity among egg sacs,
nestsandsitesusingtheG-testofSokalandRohlf Chromosomal analysis.—Both meiotic and
(1981). This statistic has a chi-square distribu- mitotic preparations were successfully obtained
tion but ismorereliablethan thetraditionalchi- from testes. Mitoticpreparationswereexamined
square test when sample sizes are small. for 58 embryos from four egg sacs in all.
ROWELL & MAIN-SEX RATIO IN A SOCIAL SPIDER 203
Table2.—SummaryofdataobtainedfrommorphologicalanalysisofadultandsubadultD.socialisspecimens.
Colonydata, heterogeneitywithinsites, maleproportionandnumberofcolonieswitha femalebias. Treatment
ofoutliersdiscussed in text. *P < 0.001.
Female bias
Hetero- Male
No. of Signifi- geneity propor-
G
Site colonies cant Present value n tion
Pemberton 26/7-1/8 Areal 10 7 9 8.03 ns 214 0.23
Pemberton 26/7-1/8 Area2 9 8 9 8.17 ns 240 0.15
Puls Rd, Torbay 26/3/89 1 1 1 29 0.28
Puls Rd, Torbay 18/4/90 4 4 4 7.82 ns 104 0.17
Puls Rd, Torbay 23/8/87 1 1 1 34 0.17
Puls Rd, Torbay 14/10 & 5/11/90 8 5 6 43.55* 213 0.26
(withoutoutliers) 5 4 5 3.536 ns 102 0.29
Curinup Rd 3/6/90 6 5 6 11.07 ns 89 0.12
Curinup Rd 14/10/90 2 0 0 0.07 ns 16 1.67
Total (pooled by site) 44 31 39 26.80* 939
(outliers removed) 39 30 38 12.35 ns 812 0.19
Figure 1 shows male meiosis, which involves onies were unusual in that they possessed resi-
1 1 bivalents and the X-chromosomes. Of im- dent clubionid predators. Consequently, the sex
portance here is the fact that two X-chromo- ratios in these collections are considered to be
somesarevisible, indicatingthatD. socialisem- non-representative ofthe adult sex ratio ofthe
ploysthe XjXjO system common to most spider population as a whole, and these colonies were
species. This was reflected in the mitotic prep- removed from further analyses.
arations from embryos (Figs. 2, 3), where indi- Intotal,38oftheremaining39coloniesshowed
vidualspossessedeither 24 or 26 chromosomes. a female bias, and in 33 of these the bias was
Clearly, the formerrepresent male embryos and statistically significant. The overall sex ratio de-
the latter females. In many ofthe preparations termined from the morphology was not signifi-
a number ofchromosome fragments or micro- cantlydifferentfrom theoverallembryonicratio
chromosomes (ranging from one to three) was (Table 3).
visible (Figs. 2, 3). ComparisonwithAnelosimuseximius.—Habit
Table 1 showsthenumbersofmaleandfemale 3 showstheresultsofcomparisonsofembryonic
embryos identified foreach egg sac analyzed. In and morphological data between D. socialis and
every case, the number offemales exceeded the A. eximius from Aviles (1986) and Aviles and
number of males, and the results were statisti- Maddison (1991). For both species the embry-
cally homogeneous among the egg sacs. The onic ratios do not differ significantly from those
pooled data deviate significantly from a 1:1 sex ofadult and subadult spiders, but a greater bias
ratio, with an overall male proportion of0.28. is apparent in the morphological data. For both
Morphology.—Table 2 shows a summary of types of data, however, the female bias is sig-
the data obtained from morphological analysis nificantly greater in A. eximius.
ofadultand subadult D. socialisspecimens. The
DISCUSSION
ratios among colonies and sites were quite uni-
form, with only five deviant colonies (one from Karyotype of Diaea socialis.—Tht chromo-
Puls Road, 14October 1990, two from thesame somenumberandsexdeterminationmechanism
locality on 5 November 1990 and both of the ofD. socialis differ from those reported for D.
CurinupRoad, 14October 1990,colonies).Since subadulta Rosenberg & Strand, the only other
these colonieswere all collected in mid-October species of the genus which has been examined
orlater,whichistowardstheendofthedispersal karyotypically (Suzuki 1952). D. subadulta pos-
phase, itisprobablethattheyrepresenttherem- sesses 26 autosomes and one X-chromosome in
nants oflarger colonies following emigration of the male and consequently a complement of26
many ofthe spiders. Moreover, two ofthe col- autosomes and two X-chromosomes in the fe-
204 THEJOURNALOFARACHNOLOGY
male can be inferred. Within D. socialis, how- Table 3.—G-test comparisons involving morpho-
ever, the chromosome number and karyotype logical and chromosomal data sets from A. eximius
form are conserved, with identical male com- (Aviles 1986,Aviles&Maddison 1991)andZ).socialis.
plements present in populations from Victoria *{P < 0.01), **(P < 0.001).
and the Australian Capital Territory, separated
from the Western Australian study site by dis- Analysis G Value
tances ofover 3000 km, with large areas ofin- A. eximius, heterogeneityamong
tervening desert (unpublished data). This con- colonies, morphology 24.108 ns
trastswithcytologicaldatafromtwootherwide- A. eximius, embryos x morphology 0.078 ns
rangingspiderspecies,DelenacanceridesandSe- D. socialis, embryos x morphology 2.602 ns
lenops australiensis Koch which show marked A. eximiusembryos x D. socialis
embryos 10.454*
geographical variation in chromosomal number
A. eximiusmorphology x D. socialis
and karyotypic form across their range (Rowell
DMR morphology 24.496**
1990, unpublished).
Sex ratio in D. socialis and A. eximius.—The
embryonic sex ratio bias in D. socialis is consis-
tent with the data from adult and subadult spi- determiningsperm, ortheycompetelesswellfor
ders. Thus it can be concluded that, as in A. ferTtoiligzaaitnioannoyfftihneerergeg.solutionwould requirethe
eximius, the bias is present before hatching.
use oftechniques capable ofdistinguishing be-
The tests for heterogeneity indicate that there
tween maleand female sperm, such as sedimen-
is a marked conservatism in the sex ratio ofD. tation rate or DNA content. Alternatively, ifan
socialis both between sites and through time, as X-linked probe were available, it might be pos-
thedataarederivedfromobservationsoverfour
sibletodeterminetheratioofthemarkertototal
years. Moreover,theredoesnotappeartobeany DNA in males, females and a sperm sample us-
relationshipbetweenmonthofcollectionandsex ingatechniqueassimpleasdotblotting. Ifmale-
ratio, except duringthe period ofdispersal, over and female-determining sperm are present in
the October/November period. This is an im- equalproportions,theratioofthemarkertototal
portantobservationbecausesuchuniformitymay DNA in male tissue would equal the ratio in the
notbeexpectediftheratioisinfluencedbysome sperm sample, and be halfthat found in female
environmental factor. For example, if the sur- tissue(because females possesstwiceas many X
vival of male embryos or the success of male chromosomes as males). If, however, the bias
determining sperm were influenced by temper- arises from a deficiency of male-determining
ature, considerable variation in ratio would be sperm,Xchromosomeswouldbepresentinmore
expected between years, oreven within one sea- than halfofthe sperm; and the ratio would be
son. intermediate between that of male and female
The mechanism involvedin producingthe fe- tissue.
male bias observed is not apparent; however, Hamilton (1967) pointed out that Fisher’s
someconclusionscanbedrawn.AsinA. eximius principle isbasedon theassumptionofpanmix-
(Aviles & Maddison 1991), few infertile D. so- ia,andthatselectionongroupswhereinbreeding
cialis eggs were observed, and certainly insuffi- within a singleclutch is the norm may favorthe
cient to make up the shortfall in males even ifit production of fewer males, thus minimizing
were assumedthatevery one representedadead competitionbetweenmaleoffspringandthecon-
male embryo. The little “infertility” that is ap- sequent wastage of reproductive effort. This
parent in this species is usually associated with would appear to be a probable explanation here
egg parasitism (BYM unpublished). In A. exi- since the life histories ofboth D. socialisandA.
miusandD. socialis, meiosisproceeds normally eximius suggest marked inbreeding. This is fur-
through to anaphase II, and so it is possible to thersupported by preliminarydata from the so-
narrow down the time at which the mechanism cial spider Delena cancerides. This species has
acts in both species to the period between the been demonstrated to be outbred by the use of
end ofmeiosis and fertilization. Since sex is de- enzyme electrophoresis (Rowell 1990), and on
termined by the chromosomal composition of the basis ofdissection ofhatchlings itwould ap-
sperm, it follows that male-determining sperm pear to have an even sex ratio (Rowell, unpub-
areeitherpresentinlowernumbersthanfemale- lished).
ROWELL& MAIN-SEX RATIO IN A SOCIAL SPIDER 205
An alternative explanation discussed in detail thedifferenceinsexratiobetweenD.socialisand
by Aviles (in press) is that, in continuously in- A. eximiusmayimplyadifferenceintheoptimal
breedinglineagesofsocialspecieswherecolonies ratio, determined by their life histories. Thus a
are initiatedby individual mated females, selec- detailedcomparisonofthelifestylesofthesespe-
tionatthecolonylevelplaysamajorrole.Future cies may shed light onthe particularfactors that
success of a given lineage is a function of the influence sex ratio.
numberofsuccessful femalesratherthantheab-
ACKNOWLEDGMENTS
solute numberofoffspring. Consistent with this
isthatinthesolitaryleaf-rollingthomisidsofthe WethankT.A.Evansforcollectionandcounts
genus Cymbacha L. Koch, which presumably of specimens from the Manjimup region. We
have an even sex ratio, dispersal occurs during would also like to thank Ian Scott and Angela
thejuvenile stage; and, asin manyspecies, there Higgins for their help with data analysis and
is likely to be a high mortality in the free-living preparation ofthe manuscript. Leticia Aviles of
stage ofthe life cycle in both sexes. Moreover, the University ofArizona provided particularly
thechancesoffree-livingfemales findinga mate helpful comments on the manuscript. The Zo-
relyonarelativelyhighdensityofmales. Incon- ology Department ofthe University ofWestern
trast, because only adult females of D. socialis Australia provided some ofthe facilities neces-
(which are assured ofa mate within the colony) sary for this work.
disperse, the biased ratio increases the proba-
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