L4 - The genetics of plant domestication and conservation

Question 1

Outline the basic studies of Mendel that introduced a foundation to genetics

What key observation could he not understand and why?

Answer 1

Gregor Mendel researched inheritance in peas (Pisum sativum)
- Observed Mendelian traits e.g. green vs yellow
- Inherited in binary manner
- Show dominant + recessive relation
- Inheritance of Mendelian ratios then seen e.g. 3:1 dominant:recessive in F2

Mendel observed segregation of unlinked traits
- Couldn’t understand linked traits resulting from genes on same chromosome
- (Tbf he didn’t know about chromosomes)

Question 2

Give the main function of meiosis and how we can use it

Answer 2

• Produces new combinations of genetic diversity
• Major tool to analyse genetic variation in research + breeding

Question 3

Why are cereals useful crops?

Which main cereal crops account for 2/3 of our calories?

Answer 3

Seeds are:
- High in protein + carbs
- Storable
- Can germinate to regenerate a crop year on year

Mainly rice, wheat, maize

Question 4

Describe the three main cereal crops giving:
1) Location + time of doestication
2) Wild progenitor
3) Genome size

Answer 4

Wheat - Triticum aestivum
1) Mesopotamia - 9,000 BC
2) Hexaploid hybrid of Pooid grasses
3) 17,000 Mb genome

Corn - Zea mays
1) Mesoamerica - 2,500 BC
2) Derived from Panicoid Teosinte
3) 2,500 Mb genome

Rice - Oryza sativa
1) China (+ other places) - 10,000 BC
2) Derived from wild rice O. rufipogon
3) 430 Mb genome

Question 5

Outline the hybridisation events that occurred in wheat

Answer 5

1) Wild diploid wheat species T. monococcum and T. searsii hybridise = tetraploid T. turgidum (both 2n = 14)

2) T. turgidum hybridises with T. tauschii (2n = 14) = hexaploid T. aestivum (2n = 42)

Question 6

Describe the difference between homologous and homeologous genes in wheat and the genetic significance of them

Answer 6

• Homologous chromosomes come from same progenitor species (e.g. A1 and A2)
• Homeologous chromosomes contain similar gene positions but from different progenitors (e.g. A1 and B1)
• Replacement of homeologues with others causes specific phenotypes (related but specify different phenotypic aspects)

Homologous chromosomes must pair together in meiosis for fertility
- depends only on Ph1 locus, arose early before tetraploid = unconscious selection?
- Ph1 contains rearranged CDK genes + ZIP4 meiosis genes
- CDK control cell cycle via phosphorylation, ZIP4 regulate crossover formation
- Ph1 likely modifies CDK expression/recombination pathway, allowing homologue recognition

Question 7

Give two more traits of modern wheat that have been selected for and are controlled by specific gene loci

Answer 7

1) brittle rachis mutations (br) = anti-shattering (good harvesting)

2) Q and tg gene mutations = soft glumes + naked grains (better threshing + flour)
- single amino acid change in AP2 TF caused important Q mutation (unconscious selection)

Question 8

Describe an example of a key mutation that contributed to the Green Revolution

Answer 8

Reduced Height (RHt) Mutations
- prevent GA-mediated destruction of DELLA proteins
- DELLA proteins inhibit growth = dwarfism
- Reduces lodging + better harvesting

Question 9

Give an example of the arms race between resistant genes and pathogens

Answer 9

Resistance to stem rust in wheat conferred by Sr31, Sr38 R genes for 50 years
- Ug99 pathogen virulent against this e.g large yield costs in Uganda
- Highlights arms race

Question 10

Give a main problem with domestication and measures taken against this

Answer 10

Creates genetic bottleneck
- Only retain small fraction of R genes + more
- Conservation of wild diversity important
- But back crossing takes a while + linkage drag