sex & mating systems Flashcards
what is the importance of sex identification
important for behavioural and ecological research + for breeding and conservation work
- The sex of an individual is a key variable for any biological project
- But there are situations where the sex of an individual cannot be determined by just looking
what are the 5 Methods for sex identification
- Morphological clues
- Surgery (laparotomy and laparoscopy)
- Cytologically
- DNA fingerprinting
- PCR amplification of a sex-linked gene
how to use Morphological clues to identify the sex of an individual
e.g. external genitalia and secondary sexual characters
BUT: not all animals are sexually dimorphic
Example: only a portion of the world’s bird species can be sexed using morphological differences + Juvenile fish cannot be sexed with external cues
how to use surgical methods to identify the sex of an individual and its disadvantages
an incision is made in the abdomen to allow direct examination of the internal sex organs
BUT:
-risky for some animals
-stressful for the subject
-time-consuming
-Requires expert
-costly
how to use Cytology methods to identify the sex of an individual and its disadvantages
examination of the sex chromosomes in the nucleus
BUT:
-need to culture the cells first
-tissue/blood samples must be of good quality (and cells must be living) to allow successful culturing (hence field sampling can be difficult)
-producing a good chromosome spread is tricky
-requires expert
how to use DNA fingerprinting to identify the sex of an individual and its disadvantages
examine minisatellite, microsatellite, RAPD or AFLP polymorphisms to find markers which are sex-specific.
BUT:
-Based mainly on polymorphisms in non-coding regions of the DNA: mutation rate is high
-Different protocols needed for different species
how to use PCR amplification of a sex-linked gene to identify the sex of an individual and its disadvantages
- DNA sequences of genes tend to be conserved across species (functional)
- Markers based on a sex- linked gene would be more likely to be useful across many species & PCR primers can be specific
- ‘Universal’ molecular sex identification using PCR is possible for mammals and birds
BUT:
-Not all organisms have sex determined by genes
-Sex-specific genes are not conserved across plants and animals
-Different markers developed for individual species for plants and for fish are still necessary
what sex-linked molecule is used to identify mammals
SRY = a gene on the Y chromosome
- Example: Whale sexes cannot be easily distinguished by observation - PCR amplification of a 147 bp fragment of SRY gene allows sexing of live individuals
what sex-linked molecule is used to identify birds
sex chromosomes = ZZ (males) and ZW (females) - Females therefore possess a unique chromosome (W) which could serve to identify them
- The chromobox-helicase-DNA-binding (CHD1) gene = a sex-linked gene for birds
- Females would have CHD1-W and males would have CHD1-Z
- PCR products are separated by size with gel electrophoresis: 2 bands (CHD-W and CHD-Z) = female / 1 band (CHD-Z) = male
Fisher’s Sex Ratio Theory suggestions and assumptions
suggests that the optimal and stable primary sex ratio (at fertilization) must be 50:50
Fisher’s theory assumes:
- Male & female parents are equally good at producing male & female offspring at all ages and sizes
- Mating is random in a large population
who suggested a new theory and what is it
Trivers & Willard (1973)
- Animals should adjust the sex ratio of their offspring in response to different environmental conditions
- Red deer example:
>A female will always breed
>A male must be large enough to defend a harem, so size provides an advantage
>Consequently, a mother in good condition who can produce a large offspring is more likely to produce a male
>A mother in poor condition who can only produce small offspring is more likely to produce females
>The flexibility of some species to modify offspring sex to maximise fitness may sometimes be a problem for conservation
why could Sexual conflict arrise
- The sex with higher reproductive cost is choosy (females in many but not all taxa)
- while the sex with more reproductive potential (males, same consideration) competes for number of mates - results in differential reproductive success
name one Direct and indirect benefit of mate choice
D = Males provide material benefits that enhance female’s survival and that of her offspring
I = Some males might have genes that confer greater fitness to the female’s offspring (e.g. parasite resistance in fish)
advantages to individuals with mate choice compared to ones without (artificially-bred individuals)
Compared to artificially-bred individuals (no mate choice), wild individuals show:
- evidence of significantly lower parasite loads
- evidence for disassortative mating (more MHC dissimilarity) - less likely to be infected
- significant increase in offspring fitness simply through more mate choice
what is the lek paradox and its hypothesis
explain why females don’t chose the same males to mate with
Hypotheses:
- The environment is constantly changing
- Male quality is determined by multiple genes
- Genetic compatibility: e.g. MHC & inbreeding avoidance
