Sex Determination in Plants

Question 1

What are the Sexes in Plants?

Answer 1

• Most plants are bisexual with flowers containing both sexes
  - male (androecium or stamens)
  - female (gynoecium or pistils)
• Some plants are also dioecious (sexes in separate plants)
  - Pistillate plant (female)
  - Staminate plant (male)
  *Approx. 5% of genera of higher plants are dioecious
  *Many agriculturally important crops are dioecious
  (eg nutmeg, pistachio, cannabis, date palm, papaya and asparagus)

Question 2

How do we determine sex in Asparagus?

Answer 2

Sex is determined by a single autosomal gene
-Staminate (MM or Mm)
-Pistillate (mm)

Staminate Pistillate
MM x Mm
M m
\ /
Mm
(male)

Question 3

How do we determine sex in maize (Zea mays)?

Answer 3

Cultivates maize is monoecious
- both sexes on the same plant
- but on different inflorescences (flowers)

Apex- carries the male inflorescence called a ‘tassel’
- the tassel has staminate flowers
Axil- carries an axillary female inflorescence called an ‘ear’
- The ear has pistillate flowers

Question 4

What are the genes that determine sex in maize?

Answer 4

There are three genes involved in sex determination:

*‘bs’ mutation
- in the homozygous recessive state (bsbs)
- results in barren stalk (no ears)
- converts the monoecious corn to a staminate plant

*‘ts’ mutation
- in the homozygous recessive state (tsts)
- converts the staminate flowers to pistillate flowers
- produces tassel seeds
- pistillate plant- both inflorescences are female

*‘sk’ mutation (silkless mutation)
- in the homozygous recessive state (sksk)
- causes the silk not to be produced in the ‘ears’
- silk are extended stigmas from the pistillate florets
- no silk- no pollination and fertilisation- no seeds
- sk mutation converts the corn plant to a functional male, since the tassel can produce pollen

Question 5

Some Examples of Sex Determination in Maize with combination of 3 genes that determine sex

Answer 5

BsBs TsTs SkSk
- normal stalk, normal tassel, normal silk
- Monoecious cultivated corn
bsbs TsTs SkSk
- Normal tassel, no ear
- Staminate plant
BsBs tsts SkSK
- Normal ears, tassel converted to female
- Pistillate plant
BsBs tsts sksk
- Same as before but no silk
- Non-functional female

Question 6

How is sex determines in Melandrium (flower)?

Answer 6

Shows sexuality similar to a XX-XY system
- staminate plants are XY
- pistillate plants are XX
*The Y chromosome is larger than the X
- X/Y ratio = 1 = male
- X only = female
Genotype X/Y ratio Sex
XYY 0.5 male
XXXYY 1.5 male
XXY 2.0 male + some intersex
XXXXY 4.0 intersex + some male

Question 7

In Melandrium different functional regions on the chromosome determine sex. How do they do this?

Answer 7

Chromosome detection studies have explained the functional regions on the X and Y chromosomes.

Delete Region-I
- perfect flowers (bisexual)
Delete Region-II
- Pistillate flowers
Delete Region-III
- Sterile staminate flowers

Question 8

How do we determine sex in pawpaw (Carica papaya)?

Answer 8

The sex gene has three alleles:
- male (M),
- hermaphrodite (M^h) (male and female)
- female (m)

Females = mm (sweet but not productive)
Males = Mm
Hermaphrodites = Mhm (sweetest and productive)

*All dominant combinations are lethal: MM, MhMh, MMh

Question 9

Summary of sex determination in Plants

Answer 9

Most plants are bisexual with 5% of genera being dioecious.

Asparagus Single gene
Maize- Three genes- bs, ts and sk
Melandrium XY system: ratio of X to Y determines sex.
Pawpaw One gene with 3 alleles.