Male adaptations and female sperm use in organismal societies of ants and bees

129 Pages·2013·3.11 MB·English

by  Marlene Stürup

FACULTY OF SCIENCE
UNIVERSITY OF COPENHAGEN

CE NTRE FOR SOCIAL EVOLUTION
DanishNationalResearchFoundation
DEPARTMENT OF BIOLOGY CentreforSocialEvolution

Male adaptations and female sperm use in
organismal societies of ants and bees

This thesis dissertation has been submitted in accordance with the requirements for the
degree of PhD, at the PhD School of The Faculty of Science, University of Copenhagen,
Denmark to be defended publicly before a panel of examiners.

by
Marlene Stürup
September 2013

Academic advisor
Prof. Jacobus J. Boomsma

Preface

This thesis is the result of a three year PhD project carried out at the
Centre for Social Evolution, Department of Biology, University of Copenhagen, under
the supervision of Prof. Jacobus (Koos) Boomsma. During my studies I spent seven
months in the lab of Dr. Boris Baer, Centre for Integrative Bee Research (CIBER) and
Centre for Evolutionary Biology, University of Western Australia. I additionally spent
a total of four months of fieldwork at the Smithsonian Tropical Research Institute in
Panama, and two weeks at the Department of Biosciences, University of Helsinki. I
was funded by a grant from the Faculty of Science, University of Copenhagen

My thesis consists of two main parts. I start with a general introduction
to my model system where I explain the idiosyncrasies of eusocial insect mating
systems in particular in the most advanced species, where colonies have organismal
properties. I also present the objectives of the thesis research, and finish with a
summary of my main results in a broader conceptual framework. The second part
consists of four chapters of original empirical work, two of which were published in
peer reviewed journals, one that is currently submitted and one that is in preparation
for submission.

Marlene Stürup
Copenhagen, September, 2013

Table of contents

Abstract English & Dansk....................................................................................9

General introduction......................................................................................13

1. Model organisms........................................................................................17
1.1. Leaf-cutting ants..............................................................................................17
1.1.1 Disease agents...............................................................................................18
1.2 Honeybees........................................................................................................19

2. Theoretical framework.............................................................................20
3. Conflicts and constraints..........................................................................21
3.1. Mate number and sperm storage.....................................................................21
3.2 Worker reproduction........................................................................................22
3.3 Male reproductive success and colony sex-ratios............................................23
3.4 Sperm use..........................................................................................................23
3.5 Cheating patrilines...........................................................................................24

4. Open questions and objectives of thesis..............................................26
5. Summary and future perspectives........................................................30
References...........................................................................................................33
Chapter 1.............................................................................................................47
Random sperm use and genetic effects on worker caste fate in Atta colombica
leaf-cutting ants
Chapter 2............................................................................................................61
Sperm mixing in the polyandrous leaf-cutting ant Acromyrmex echinatior
Chapter 3............................................................................................................85
When every sperm counts: factors affecting male fertility in the honeybee Apis
mellifera

Chapter 4............................................................................................................95
Selection for maximal sperm survival makes leaf-cutting ant males unfit for
independent life
Acknowledgements........................................................................................117
Curriculum vitae............................................................................................121
Pictures..............................................................................................................127

English & Dansk
Abstract

Abstract

The males of the most advanced eusocial Hymenoptera live short
protected lives inside colonies where climatic conditions are meticulously controlled,
and where disease pressure is low due to the social immunity provided by their
worker sisters. Mature males only leave the colony to mate and die soon after,
whereas queens can live for years without ever re-mating but only using the stored
sperm they have received from one or several males. The number of live sperm
received and stored during mating therefore ultimately limits queen lifetime
reproductive success, and males are consequently under strong selection to produce
high quality ejaculates.
In multiply mated queens, ejaculates from different males arrive as
discrete units, but genetic diversity among workers has been documented to provide
several colony benefits and this diversity can only be maximized if sperm becomes
completely mixed after storage. Queens are also expected to use all the sperm they
receive and not bias sperm use away from certain males, yet sperm mixing and sperm
use in eusocial insects is poorly investigated. Using molecular techniques, I quantified
the relative ejaculate contributions from males to the queen’s sperm stores and
compared this with paternity shares in offspring. I also examined whether patriline
distributions in worker cohorts remain stable over time, which is expected to yield the
highest colony benefits in terms of worker genetic variation.
Selection for extreme sperm viability in males is likely costly and
expected to trade off with investments into other traits. I tested this by measuring
sperm viability in honeybee males after exposure to mild external stress factors.
Additionally, I infected leaf-cutting ant males with a pathogenic fungus and measured
their subsequent immune response and sperm viability. I hypothesise that male
fertility is easily compromised but that male immune defences are not under selection
to respond to challenges that they are unlikely to experience under natural conditions.
I found that sperm use in a long-lived eusocial ant species follows a ‘fair
raffle’, where paternity shares in offspring can be accurately predicted from relative
number of sperm stored. Furthermore, sperm was completely mixed inside the
spermatheca and sperm used is highly consistent over time in leaf-cutting ants,
supporting expectations if polyandrous queens are to obtain the highest maximal
fitness benefits through a high degree of colony genetic diversity.
My results support that males of advanced eusocial Hymenoptera are
under strong selection to perform a very specialized set of tasks and that they might be
operating at their physiological limit. This indicates that in the most organismal
eusocial societies, males have become completely dependent on the protective
environment in which they are raised and are not adapted to any significant form of
independent life.

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Resumé

Hannerne hos de mest avancerede arter af eusociale årevingede insekter
lever korte beskyttede liv i kolonier hvor klimatiske forhold er nøje kontrolleret og
hvor sygdomspresset er lavt takket være den sociale immunitet deres arbejder søstre
yder. Seksuelt modne hanner forlader kolonien for at parre sig hvorefter de dør,
hvorimod dronninger kan leve i årevis uden at parre sig igen men kun udnytte den sæd
de har modtaget og opbevaret fra én eller flere hanner. Antallet af sædceller
dronningerne opbevarer under parring vil derfor være den begrænsende faktor for
deres reproduktive succes, hvilket betyder at hanner er under stærkt selektionspres for
at producere sæd af høj kvalitet.
Hos flerparrede dronninger modtages sæden fra flere hanner som
diskrete enheder, men genetisk diversitet hos arbejderne i kolonien medfører flere
fordele som kun udnyttes fuldt ud hvis sæden bliver blandet fuldstændig når den
bliver opbevaret. Dronningerne forventes desuden at udnytte al den sæd de modtager
og ikke forfordele bestemte hanner, men studier over sædopbevaring og -brug hos
sociale insekter er sparsomme. Ved at bruge molekylære teknikker, kvantificerede jeg
den relative andel af sæd fra forskellige hanner i dronningens sædgemme og
sammenlignede dette med fordelingen af fædre i dronningens afkom. Jeg undersøgte
også om den relative fordeling af fadderskab i kohorter af arbejdere er stabil over tid,
hvilket er forventet hvis kolonien skal drage nytte af fordelene ved genetisk variation
blandt arbejdere.
Selektion for ekstrem sædoverlevelse hos hanner forventes at koste
resurser der således ikke kan allokeres andetstedshen. Jeg testede dette ved at måle
sædoverlevelse hos honningbi hanner efter de havde været udsat for milde eksterne
stress faktorer. Ydermere inficerede jeg hanner af bladskæremyrer med en patogen
svamp og målte derefter deres immunreaktion samt sædoverlevelse. Jeg forventer at
hannernes fertilitet vil falde men at deres immunforsvar ikke er under selektion for at
reagere på udfordringer som hanner under naturlige omstændigheder ikke er udsat for.
Jeg fandt at fordelingen af fædre i afkom nøjagtigt svarede til de
forskellige hanners relative bidrag til dronningernes sædgemme i eusociale myrerarter
hvor dronningerne lever længe. Ydermere blev sæden hos bladskæremyrer
fuldstændig blandet efter den var opbevaret i dronningens sædgemme, og brugen af
sæd til befrugtninger var konsistent over tid, hvilket svarer til forventningerne hvis
dronningerne skal drage maksimale fordele af genetisk diversitet i kolonien.
Mine resultater understøtter at hannerne i avancerede arter af eusociale
årevingede insekter er under stærkt selektionspres for at udføre nogle få meget
specialiserede opgaver og at de potentielt arbejder på kanten af deres fysiologiske
formåen. Dette tyder på, at i arter af eusociale insekter hvor kolonien har egenskaber
der er sammenlignelige med en organisme, er hannerne blevet så afhængige af de
beskyttede forhold de vokser op under at de ikke længere er i stand til at leve
uafhængige liv.
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