Costs of sex Sex understanding its biological significance 1 Males dilute females genetic contribution the couple is the unit of reproduction appreciating how genetics was used to understand how it is determined according to Jacob Bronowski in The Ascent of Man 1973 Mendel himself was inspired by the clear cut difference between males and females and the 1 1 sex ratio Benefits of sex 1 Reduces mutational load escape Muller s ratchet irreversible loss of genes perhaps males particularly useful 2 Seeking a mate and mating takes time and energy and is dangerous 3 Sexual conflicts arise remember the Haig hypothesis for imprinting 4 Sex and its consequence recombination break up winning gene teams Sex determination genes determine two qualitatively different things a distinction not often appreciated even by those who study the genetic programming of sex population sex ratio sexual dimorphism developmental differences rationale for maladaptations from sexual selection 2 Free good mutations from bad genetic backgrounds 3 Help to keep ahead of parasites there is no optimal genotype in the real world An extreme example of sexual dimorphism relevant variables for ESD Host Bonellia Bonellia viridis Female 100 mm Male 1 mm Temperature turtles alligators larva lands on rock larva lands on adult female ESD environmental sex determination Neighbor density parasitic wasps Presence of male tropical fish vs GSD genotypic sex determination Segregation of alleles genes determines sex best for generating 1 1 sex ratios 1 Known for fruit flies XX females XY males apparant paradox but what really determines fly sex Since females are rate limiting for reproduction why see 1 1 sex ratio so often as usual Darwin had the answer first Calvin Bridges 1916 w w white eyed In the aggregate both sexes contribute equally to the next generation every female needs a male expected hence any minority sex on average will make a disproportionate contribution per individual Natural selection will favor generation of the minority sex At 1 1 no minority sex for fruit flies exceptions primary Females X Males red eyed w Y w Y XY white sons white daughers fertile red sons sterile x red X Y progeny are secondary exceptions XX w w red daughters XXY X O xxx o Y die white daughers fertile red sons fertile XXY XY Y XX females XY males normal XX females XY males What about X chromosome number matters abnormal XXY females XO males Sex chromosome difference CAUSES triggers different sexual development Y chromosome does not detemine sex but is required for male fertility absolute number 1 male 2 or more female odd vs even paired XX X male number relative to ploidy non sex chromosomes X AA male but X A female X chromosome number determines sex again genetic exceptions to the rule provide the answer autosomal genes px bw bw sp Females X Males px sp X A 0 5 male XX AA expected PROGENY Parental types px sp Nonparental types px sp recombinant XXX 6 5 cM AAA ALSO one unusually large female X px bw sp Male 1 Three not two parental types recovered px bw bw sp px sp X AA XX Y AAA X A 1 female X A 0 67 intersex XXX AAA X A 1 female large XA X A 1 dead female 2 many intersexual XXY sterile progeny AAA 3 normal and jumbo females 2 GENETIC MOSAICS X AA X chromosome loss generates gynandromorphs XX AA zygote XXAA cells X AA cells XXAA X AA Female Male X A 0 5 male XX AA XXAA zygote XXAA cells XA cells XX Y AAA X A 1 female X A 0 67 intersex loss of an entire haploid set XXX AAA X A 1 female large XXAA X A Female Female XA X A 1 dead female GSD by X A ratio balance XA never reaches adult stage but mosaics do The worm The worm XX self fertilizing hermaphrodite XX self fertilizing hermaphrodite XO male heterogametic sex XO male heterogametic sex Origin of males XX AAA X A 0 67 male 1 Spontaneous X chromosome nondisjunction rare to make O eggs X self sperm XO male XXX AAAA X A 0 75 hermaphrodite GSD by X A ratio 2 Mating outcross of hermaphrodite to male X eggs join with X or O male sperm 50 50 HUMANS HOUSE FLIES XX female XY male XXY Kleinfeler Syndrome sterile male 1 1000 men XO Turner Syndrome sterile female 1 2000 5000 m m female M m male GSD by dominant masculinizing allele M one of three different GSD systems in the same species GSD by Active Y dominant masculinizer 3 Birds moths and butterflies ZZ male ZW female female is the heterogametic sex compare XY males GSD by feminizing W or Z A 20 of all animals use a very different GSD system Eggs fertilized Queens females or workers sterile Diploid royal jelly Eggs not fertilized Drones males Haploid GSD by haplodiploid system But is the relevant variable ploidy Let s encourage inbreeding among the honeybees increased homozygosity suddenly DIPLOID MALES a1 a2 heterozygotes females queens and workers a1 or a 1 a1 hemizygotes and homozygotes males fertilization a1 a2 Queen X a1 Drone a1 a1 a 2 a1 diploid drones GSD by a multiple allele system highly polymorphic sex gene many alleles 4
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