8 - Sex Allocation Flashcards
What is sex allocation?
Allocation of resourves to male v female reproduction
Depends on breeding system and mechanism of reproduction
Differs in dioecious and hermaphroditic individuals
Hermaphrodites can be simultaneous or sequential
What is the Fidher condition?
Established in 1930
Each offspring has one father and mother
Selection leads to unbiased sex ratio
M and F make equal genetic contributions
What species use different types of chromosomal sex determination?
Mammals, reptiles and some insects: F XX, M XY
Birds, reptiles and insects: F ZW, M ZZ
Some insects have: F XX, M XO (Haplodiploid, no Y chromosome)
Describe temperature driven sex dertmination, with examples
Temp during embryonic stage
Pattern 1A: males hatch below temperature zone, eg. red eared slider turtle
Pattern 1B: converse
Pattern 2: males hatch at intermediate temperatures, females at extremes, eg. crocodillians
More important for males to develop in good conditions, as bad conditions affect males more than females
What is the Dusing-Fisher theory of investment?
Sex ratio: n(males)/n(females)
Fisher based his 1930s arguments on earlier ones by Dusing and Darwin
Natural selection favours = investment, therefore m=f reproductive value
If there is an excess of females, each female has less than one mate, therefore male value increases and selection favours males, ie. if costs aren;’t equal, selection favours the sex investing least
A 50% ratio achieved at population level
Allows predictions of equal investment and frequency dependency
What assumptions do we make under the Dusing-Fisher theory?
No interaction between relatives
No differential efects of environment on m and f
Parental control of sex ratio
50% ratio doesn’t occur when not met
How does resource competition bias sex ratios?
Male based sex ratio in Greater Galago
Males disperse, females do not
Mothers and daughters compete over resources
Local resource competition decreases value of daughter as it has to compete
Increases value of males selected for
Hamilton noticed correlation: female biased sex ratios and more competition between brothers so one unsuccessful brother would be wasted
Unknown why rule doesn’t apply here
Nasonia - females assess local mate competition by sensing how many eggs other females habe made and adjusting sex ratio in own accordingly
How does local resource enhacement affect sex ratio?
In some cooperatively breeding species, offspring of one sex are more likely to be helpers, but not always most common sex
Depends on environmental factors
eg. Komandeur found in seyshelles warbler, high quality territories are female biased 9:1, poor are male biased 8:1
Local resource enhancement and local resource competition occurs here
How does maternal condition impact sex ratio?
Trivers and Willard, 1973 - sex ratio adjusted depending on conditions
Rum deer - mothers in good condition can afford to produce larger offspring
Size hugely impacts reproductive success in males, but not females
Deer in good condition produce more males as size is more valuble to males
Mechanism unknown
Males more costly to produce
Describe how environmental sex determination works in G. dubeni
Atlantic coastal shrimp
Males produced when there are more than 14 hours of daylight
Reproduce apr-aug
Non-overlapping generations
Females therefore produced later in the season
Males are bigger at breeding time than females
Size at breeding dependent on time available to growth prior to breeding
Large females lay more eggs, large males have more mating success
Size and fitness correlation more important in males
How does environmental sex determination work in bearded dragons?
Evolution ought to progresss from genetic to environmental determination
Dragon: ZZ males, ZW females
As temperature increases in wild, 20% of ZZ are females
Solved by reduced frequency of W chromosome
Climate change leads to loss of W
How does sex change occur in animals?
If fitness if a sex changes with age, sex change, eg. Bluehead wrasse turns male
Males set up territory and females chose males w largest territory
Small males 2x daily, large males 40x
If you remove large males, large females become males and adopt colours
What can distort sex ratios?
Selfish genetic elements gain advantage over individuals
H. bolina contains wolbacia bacteria, which is passed on maternally through eggs and kills males
This frees up resources for useful females
Less common due to Parliament of genes
What is the parliament of genes?
Genes act selfishly, but if this hurts others they will combine to supress it
Summarise Charnov and Bull, 1977
Sex usually determined pre conception, some after
When organisms fitness is compromised and can’t change environment it is determined after
Selection favours parental ability to manipulate sex for highest fitness
Conditions may include mate competition, differential resource value, differential predation
If parent can evaluate environment, this can occur eg. Hymenoptera
Nematodes: sex ratio determined inside host, high population density = males, low = females
Large size more important
Dioecious plants - females larger in the sun, males in shade
Hermaphrodites often change sex due to size
