Sex Evolution and Behavior 3

Sexual dimorphism

Biological differences between males and females in body size, composition, behavior, hormonal profiles, and disease transmission routes, generated by selection for reproduction

Sex bias in medical research

Strong historical bias toward studying males while treating females as "anomalies," leading to lack of understanding of disease manifestation and medication responses in women

Intralocus sexual conflict

When natural and sexual selection favor different fitness optima in each sex, with genes favored in one sex being detrimental when expressed in the other due to shared genome

Genomic tug-of-war

The consequence of imperfect sex-specific gene expression in intralocus sexual conflict, where genes cannot be optimized for both sexes simultaneously

Cryptic female choice

Female mechanism in lizards where sperm from large males are directed to fertilize male offspring while sperm from small males fertilize female offspring, resolving intralocus conflict over body size

Parent-offspring genetic conflict

Conflict between paternal and maternal genes over fetal growth and resource allocation during pregnancy, where paternal genes encourage growth and maternal genes moderate it

Placental invasion

Process where placental tissue invades maternal uterine arteries, reducing arterial plasticity and promoting blood supply to the placenta, can lead to preeclampsia

Gestational diabetes

Condition during pregnancy where insulin resistance occurs due to placental hormones affecting maternal insulin sensitivity, with pancreas unable to produce sufficient insulin

Preeclampsia

High blood pressure condition in pregnancy resulting from placental tissue invasion of uterine arteries; can be early-onset (inadequate invasion) or late-onset (maternal cardiovascular defects)

Genomic imprinting

Epigenetic modification of genes dependent on parental heritage, where DNA sequence unchanged but chemical modification (methylation) silences one allele based on parent of origin

Imprinted genes

Genes where only one allele is expressed; which allele depends on whether it was inherited from mother or father

Methylation

Chemical modification where methyl groups attach to DNA segments, controlling whether DNA can be expressed; mechanism of genomic imprinting

Genetic conflict hypothesis (imprinting)

Theory that genomic imprinting evolved due to competition between male and female parents for maternal resources during pregnancy

Paternal imprinting

When genes inherited from father are silenced by methylation, typically promoting growth; opposite of maternal imprinting

Maternal imprinting

When genes inherited from mother are silenced by methylation, typically suppressing growth; opposite of paternal imprinting

Selfish genetic elements

Stretches of genome that gain increased transmission relative to rest of genome but are selectively neutral or detrimental to organismal fitness

Intragenomic conflict

Conflict within a genome where genes or stretches of DNA function to increase their own transmission at the detriment of other genes in the same genome

Genetic drive

When a genetic element is transmitted at higher rates to progeny than expected by Mendelian inheritance (not 50:50)

Transposable elements

DNA sequences able to move to new positions in the genome via cut-and-paste mechanism; among most successful selfish genetic elements; 45% of human genome

Biased gene converters

Selfish genetic elements that preferentially insert into homologous uninserted sites using homing endonucleases; likely originated from DNA repair processes

Meiotic drivers

Selfish genetic elements achieving drive through preferential segregation to functional gamete (egg in females) during gametogenesis

Gamete killers

Selfish genetic elements that selectively eliminate or disrupt gametes lacking the driving element, increasing transmission

B chromosomes

Additional chromosome sets not required for viability or fertility but persist in population via self-propagation; occur in 15% of eukaryotes

Selfish mitochondria

Maternal-inherited mitochondria in conflict with biparentally-inherited nuclear genes; example: cytoplasmic male sterility

Cytoplasmic male sterility

Loss of male fertility in hermaphroditic plants due to selfish mitochondria affecting resource allocation to female reproductive functions

Restorer genes

Nuclear genes evolved to suppress effects of cytoplasmic male sterility genes, creating evolutionary arms race

Two rules of selfish genetic elements (Rule 1)

Spread of SGEs requires sex and outbreeding; sexual reproduction places SGEs in new genomes; inbreeding reduces spread via decreased heterozygosity

Two rules of selfish genetic elements (Rule 2)

Presence of SGEs revealed in hybrids not population crosses; SGEs reach fixation rapidly; hybrids inherit SGE without suppressors, revealing phenotypic effects

Gonotaxis

Movement of genetic elements preferentially toward germline away from somatic cells; example mechanism of genetic drive

Segregation distorters

Selfish genetic elements using gonotaxis; example: maize chromosome knobs that pull themselves along spindle during female meiosis

P element (Drosophila)

Transposable element that causes reproductive isolation; males with P element crossed to females without show reduced fitness offspring

Interlocus sexual conflict

When male adaptation increasing sexual selection success simultaneously reduces female lifetime fecundity, with females evolving counter-adaptations

Sexually antagonistic coevolution

Perpetual evolutionary arms race between male traits harming females and female counter-adaptations, driven by interlocus sexual conflict

Sexual cannibalism

Predation of one sex during or after mating; is sexual conflict only if cannibalized sex loses fitness through lost future reproduction, not if benefits outweigh future opportunities

Male infanticide

Male strategy of killing offspring to induce female receptivity; occurs in lions and some spiders to increase paternity certainty

Cryptic female choice (in conflict context)

Female mechanism to allocate sperm and choose paternity allocation to counteract male-imposed harm

Sperm-egg interactions

Fast-evolving gamete recognition system where sperm less choosy than egg due to hybrid fitness potential and sperm competition in external fertilizers

Wolbachia

Common intracellular bacterium infecting approximately 2/3 of world's arthropods; transmitted maternally only via egg infection

Cytoplasmic incompatibility (Wolbachia)

Reproductive-manipulating mechanism where infected males can only reproduce with infected females; uninfected females produce early embryonic death

Wolbachia pest control

Strategy using cytoplasmic incompatibility to suppress arthropod pest populations; example: Asian tiger mosquito control with 200M released males achieving 94% reduction

Genome size variation (C-value)

Extraordinary variation in genome size across species (7000-fold in animals, 24000-fold in plants), poorly correlated with gene number or organismal complexity; partly due to SGEs

Reproductive isolation

Evolutionary consequence of SGEs where coevolution between elements and suppressors creates incompatibilities preventing successful reproduction between populations

Transposon repression

Host mechanism to suppress transposable element activity using small RNAs and silencing pathways; co-evolves rapidly with transposon activity (Red Queen Hypothesis)

Hybrid incompatibility

Reduced fitness in offspring from crosses between populations due to mismatches between SGEs and their suppressors

Gene duplication (SGE consequence)

Transposons cause gene duplications allowing functional divergence since original gene remains active, releasing new duplicate from strong selection

Disruptive transposition

Harmful transposon insertion that disrupts critical gene function; has been linked to cancer and hemophilia in humans

Beneficial transposition

Rare cases where transposable element insertion creates adaptive trait; examples in Drosophila and dogs show adaptive evolution associated with TE insertions

Anisogamy

Difference in gamete size between sexes (large egg, small sperm), generating intrinsic parental investment conflict integral to sexual selection

NIH mandate (1993)

Federal requirement for inclusion of females and people from different ethnic/racial groups in medical research following historical exclusion

Human placental invasiveness

Humans have most invasive placentas of any mammal, correlating with increased prevalence of preeclampsia

T-Cell Lymphotrophic Virus Type 1

Virus more likely to progress to adult T-cell leukemia in Japanese men than women; transmission route (breastfeeding duration) explains differential outcomes

Epigenetic modification

Chemical alteration of DNA or chromatin structure without changing DNA sequence; basis of genomic imprinting

Germline reprogramming (imprinting)

Week 11: PGCs retain <10% methylation; Week 19: remethylation begins; resets imprinting tags each generation

Lion male infanticide

New males kill cubs to make females unreceptive for current offspring, inducing receptivity for the new male's offspring

Stegodyphus lineatus (spider)

Spiders where males steal ~33% of egg sacks, forcing females to remate and lay replacement clutches, increasing male fertilization probability

Genital morphology (sexual conflict)

Males evolving structures to harm females into terminal investment; females evolving protective traits (interlocus sexual conflict)

Seminal fluid proteins

Male-produced proteins that upregulate female egg-laying and suppress remating, but shorten female lifespan (interlocus sexual conflict example)

Drosophila P elements

Transposable elements causing reproductive isolation; phenotype revealed only in hybrids (males with P × females without show reduced fitness)

Maize centromere knobs

Segregation distorters where knobs on maize chromosomes act as centromeres during female meiosis, pulling themselves into egg

Stalk-eyed flies sex-ratio distorters

Males carrying distorters have reduced sperm competitiveness; long stalks indicate genetic suppressors; females preferentially mate with suppressor males to ensure both sons and daughters

Polyandry evolution (SGE-driven)

Female multiple mating increases when males carry selfish genetic elements, allowing females to avoid passing elements to offspring while maintaining sperm competition

Killer gene K (meiotic drive)

Gamete killer element selective for non-K bearing sperm, increasing K-bearing sperm fertilization probability

C-value paradox

Lack of correlation between genome size and gene number or organismal complexity; explained partly by variable SGE content across species

Homing endonucleases

Endonucleases used by biased gene converters to cleave specific DNA sequences and repair with their own copy; basis for CRISPR-Cas9 technology

CRISPR gene drive

Artificial homing endonuclease system using CRISPR to produce guide RNAs cutting target genes, with homologous flanking sequences allowing construct insertion

Infection of egg but not sperm (Wolbachia)

Wolbachia maternal transmission mechanism; infected egg cytoplasm passes bacteria to offspring while sperm lacks cytoplasmic transmission

Wolbachia feminization

Wolbachia effect of converting male embryos to females; reproductive manipulation mechanism

Wolbachia fecundity reduction

Wolbachia ability to reduce female fertility as part of reproduction-manipulating strategy

Dengue virus prevention (Wolbachia)

Wolbachia prevents Dengue virus replication inside host cells, providing disease control benefits beyond cytoplasmic incompatibility

Somatic and germ cell transmission (Wolbachia)

Wolbachia transmits through both somatic cells and germline, replicating as host cells replicate

Lion-tiger hybrids (imprinting)

Offspring phenotype depends on species of mother due to different imprinting patterns between species; demonstrates rapid evolution of imprinting

Calico cats (imprinting)

Example of parent-of-origin gene expression effects due to genomic imprinting and X-chromosome inactivation

Placental lactogen (hPL)

Placental hormone affecting maternal insulin sensitivity and glucose metabolism, contributing to gestational diabetes risk

Hormonal manipulation (seminal fluid)

Male seminal fluid proteins manipulate female physiology to increase egg-laying and prevent remating, reducing female lifespan

Pre-eclampsia early-onset

Occurs when placental invasion inadequate, causing placental stress and deprived blood flow; driven by fetal genes

Pre-eclampsia late-onset

Occurs after 34 weeks due to maternal cardiovascular defects and impaired detoxification of metabolic byproducts

Human hip width compromise

Example of intralocus sexual conflict solution where hip width is compromise between male and female optima

Fruit flies (intralocus conflict)

No genomic conflict in juveniles but conflict in adults; sex-specific gene expression becomes stage-specific

Lizard testosterone expression

Testosterone at sexual maturity alters size-related gene expression in males and females, resolving intralocus conflict over body size

Transposition rate (self-fertilizers)

Greater linkage disequilibrium in self-fertilizing organisms selects for reduced transposition rates due to SGE-host linkage

Antibiotic resistance plasmids

Beneficial selfish genetic elements in bacteria; plasmids carry antibiotic resistance genes providing survival advantage

Inversion sequences (t haplotype)

Structural feature of t haplotype in mice preventing linkage disruption with resistance allele; spans >1/3 of chromosome 17

T haplotype mice

Ancient (~3 million years) selfish genetic element killing sperm lacking it; t/t homozygotes die in utero; females avoid incompatible males

Hybrid sterility

Reproductive isolation mechanism resulting from SGE coevolution incompatibilities between populations or species

Sex determination (Wolbachia)

Some insects use parent-of-origin gene expression affecting sex determination, potentially arising from genomic imprinting

Kinship theory (insects)

Hypothesis explaining imprinting evolution in social insects where worker behavior influenced by maternal genes and haplodiploid relatedness

Transposon repeat sequences

Imprinted genes cluster in areas with transposon repeat sequences, suggesting transposons may be prerequisite for imprinting

Endosperm imprinting (plants)

Genomic imprinting occurs in plant embryo and endosperm (nourishing tissue) to control nutrient flow from mother to offspring

Plant embryo imprinting

Maternal inheritance in plants creates imprinting conflict similar to mammalian parent-offspring conflict

Transposon control (plants)

Imprinting may regulate transposon activity in gametes, with imprinted genes in transposon repeat areas

Sexual conflict

Conflict over evolutionary interests of males and females regarding survival and reproduction where optimal outcomes differ

Intralocus vs. interlocus sexual conflict

Intralocus occurs at same locus with shared genome effects; interlocus occurs at different interacting loci with sexually antagonistic coevolution

Negative genetic correlation

When selection for increased value in males leads to decreased value in females (and vice versa) due to intralocus conflict

Genome-wide demethylation

Early reprogramming phase where most methylation erased during germline development

Remethylation

Re-establishment of imprinting marks on specific genes during germline development (begins week 19 in humans)

Non-sequence-based inheritance

Inheritance through epigenetic mechanisms like methylation rather than DNA sequence changes

Imprinting required for mammals

Genomic imprinting is essential for normal mammalian development; showed to evolve 150 MYA

Imprinting domains

Clustered regions of genome containing multiple imprinted genes, suggesting coordinated regulation

Imprinted gene diseases

Diseases resulting from mutations in imprinted genes; recessive alleles expressed if dominant silenced; examples vary by imprinting pattern

Sex ratio distortion

Skewed offspring sex ratios resulting from meiotic drivers on sex chromosomes; can cause population extinction