Sex chromosomes in plants

Question 1

What are the main types of plant sexual systems?

Answer 1

Monomorphic Systems (one flower type per population):

• Monocliny (~85%): Hermaphroditic flowers (both male & female organs).
• Gynomonoecy: Female and bisexual flowers on the same plant.
• Andromonoecy: Male and bisexual flowers on the same plant.
• Monoecy (6–7%): Separate male and female flowers on the same plant.

Dimorphic Systems (two flower types per population):

• Distyly (<1%): Two morphs differing in stamen/style lengths.
• Gynodioecy (<1%): Coexistence of female plants & hermaphrodites.
• Dioecy (5–6%): Separate male & female plants.
• Androdioecy (<1%): Coexistence of male plants & hermaphrodites.

Question 2

Evolution of dioecyfrom hermaphroditism?

Answer 2

Evolution often occurs through intermediate steps like gynodioecy or monoecy.

Step 1: A male-sterile mutation spreads if selfing causes inbreeding depression, making outcrossing more beneficial.

Step 2: A female-suppressing mutation spreads if it allows better resource allocation to pollen production, when tehre is enough female individuals.

Step 3: Recombination suppression ensures sex-determining genes remain linked on the Y/W chromosome. (Two-locus model)

Question 3

What are the key genes involved in plant sex determination?

Answer 3

SOFF (Suppressor of Female Function): Found in Asparagus officinalis, essential for male development.

aspTDF1: A tapetum-specific gene required for male fertility in Asparagus. Taptum needed in Pollen development

In Asparagus two-locus model. SOFF and TDF1 are Y-linked Sex DetermInation Region (Y- SDR)

Shy Girl (SyGl): A female-suppressing factor on the Y chromosome in Kiwi, expressed in carpel primordia. Mutant makes nornal pollen but is female sterile. (repressor of cytokinin signaling)

Friendly Boy (FrBy): A male-fertility factor on the Y chromosome in Kiwi. In mutant the tapetum does not develop like in the control plants.

ARR17: A single-gene sex switch in Poplar (Populus)—its suppression leads to male development.

GPAT3, CYP703, LOG: Sex-determining genes in date palm, affecting male fertility and female organ development.

Question 4

What processes contribute to the degeneration of a non-recombining, permanently heterozygous region.

Answer 4

1) The effective population size (Ne) of a Y-chromosome is only ¼ of that of a diploid autosomal region/chromosome. Therefore, the effect of drift relative to that of selection is increased.

2) Suppressed recombination leads to Hill-Robertson interference.

Question 5

Why is dioecy relatively rare in plants?

Answer 5

Only 5–6% of angiosperms are dioecious.

• Trade-offs: Male-only plants do not produce seeds, reducing their reproductive success.
• Outcrossing vs. Reproductive Assurance:Hermaphrodites can self-fertilize if needed, whereas dioecious plants require a pollinator.

Resource allocation: Male and female plants might evolve different growth patterns due to differences in reproductive investment.

Question 6

What are plant sex chromosomes?

Answer 6

• Chromosomes that determine male or female development.
• They evolve from autosomes and may have suppressed recombination.
• Found in dioecious species (e.g., asparagus, kiwi, poplar, persimmon)

Question 7

What are the two main models of sex determination in plants?

Answer 7

Single-locus model: One dominant gene determines sex (e.g., poplar, persimmon).

Two-locus model: Two linked genes control male vs. female traits (e.g., kiwi, asparagus, date palm).

Question 8

How does recombination suppression evolve on sex chromosomes?.

Answer 8

Ensures that sex-determining genes stay linked.

Possible mechanisms:
* Chromosomal inversions block recombination.
* Hemizygous regions (genes only on Y/W, missing on X/Z).
* Recombination cold spots or heterochiasmy (lower recombination rates in males).
* Over time, this creates evolutionary strata, expanding non-recombining regions

Question 9

Why does the Y/W chromosome degenerate over time?

Answer 9

Lack of recombination prevents efficient selection.

Key processes:
* Muller’s Ratchet: Accumulation of slightly harmful mutations.
* Hill-Robertson interference: Reduced selection efficiency.
* Insertion of repetitive elements & transposable elements.

In some cases, dosage compensation evolves to counteract gene loss.