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Point: Very different types of plants show a range of sex determination mechanisms

Sexual determination in plants

The degree of outcrossing ranges from Complete (Dioecy)…………moderate (monoecy) …………low (hermaphrodites)
Point: Very different types of plants show a range of sex determination mechanisms

outcrossing in a hermaphrodite
Point: Even hermaphrodites may engage in gene flow - an example of anther extrusion

90% of angiosperms are hermaphrodites - they have perfect flowers

The benefits of outcrossing often outweigh the drawbacks.

Pros

• Avoids inbreeding depression
• Promotes heterozygosity
• Promotes genetic variability

Cons

• Environmental conditions may limit pollen flow
• "Cost" of having two parents, only one of which bears seed

Therefore, even in the case of perfect flowers, there are ways of encouraging, or even requiring, outcrossing.

1. Self-incompatibility __________

Example: Corylus spp. (monoecious)

Germplasm; More on incompatibility; characterization of self-compatibilty in a self-incompatible species
Point: SI is under genetic control (multiple alleles per locus - only two possible in a diploid individual. Mutants may be self-compatible.


FYI example: Brassica spp. (perfect)

Image

More on incompatibility
Point: Read (at least) first three paragraphs of Nasrallah

2. Male sterility ___________

Example: Cytoplasmic male sterility in Zea mays (monoecious)

Mitochondrial / nuclear gene interaction

Pleiotropic effects: Sterility and disease susceptibility

Additional readings on the T cytoplasm

• General overview
  Point: Consequences of pleiotropy and importance of genetic diversity
  

• Technical review: Leavings, C. 1990. Science 50: 942 - 947
  Point: Pleiotropy and a mitochondrial gene; read (at least) first three paragraphs
  
  

Mechanisms to promote outcrossing - imperfect flowers

The basis of sex determination is selective abortion of flower organs
Point: They "basal condition" is hermaphroditic; different species differentiate at different times

• Monoecy___________
  
  
• Dioecy ___________

Details on monoecy

• Example: Zea mays

  • Plant achieves a vegetative to reproductive transition

  • Selective elimination of pistils in tassel florets and stamens in ear florets

  • Mutants cause variation in these patterns – can get pistils in tassels and stamens in ears

  • A single gene can determine if a plant will be a hermaphrodite or monoecious
    Point: Read first three (at lest) paragraphs: the role of mutation is sex determination
    
    
    
    • A more complex example: Cucumis sativa - can have unisexual and perfect flowers on the same plant.

    FYI only

Details on dioecy

• Distinguishing feature: sex chromosomes in plants
  Point: Read (at least) first two pages
  • Parallels to XY (mammals) and X:A ratio (Drosophila)
    
    
  • Example: Cannabis sativa (XY)
    Point: FYI details
    
    • No evidence of rudimentary or vestigial forms of flowers of “missing” sex
    • Female: 2n = 18 + XX
    • Male: 2n = 18 + XY
      
      
  • Example: Asparagus officinalis
    Point: Using doubled haploids to prod cue YY males
    
    • Male and female flower buds indistinguishable up until meiosis – then get arrest of pollen or embryo sac formation
    • Males XY
      • There are also YY males
    • Males have increased vigor and therefore desired for commerce. YY x XX to produce all male XY crops

• Evolution of sex chromosomes from autosomes

  • Accumulation of sex determining genes
  • Prevent recombination between sex determining genes – often by accumulation of inversions
  • Theoretically leads to leads to degeneracy of Y – except for “maleness” genes. But Lemos et al. (2008) report that in Drosophila the Y chromosome has genetic
    elements that regulate hundreds of autosomal genes!
    Point: Y not inert
  • Create ~ equal numbers of male and female offspring
  • Y chromosomes are in a permanent haploid state

• An example of sex chromosome evolution in papaya (Carica papaya)
  Point: Read (at least) abstract of Liu et al.
  

• 1 locus with 3 alleles: M, M^h, m
  • MM, MM^h, M^hM^h are lethal
  • M^hM is male
  • M^hm is hermaphrodite
  • mm is female
• M^hm (pyriform) fruits are preferred; mm are spherical
• Can’t sex trees until flowering, so markers for M alleles would be useful

Useful Readings:

• Dellaporta and Calderon-Urrea. 1993. Sex determination in flowering plants. The Plant Cell. 5: 1241-1251
• Tanurdzic and Banks. 2004. Sex-determining mechanisms in land plants. The Plant Cell. 16:S61-S71.
  
  

Useful Links:

• Atlas of Plant Anatomy
• NRCS Plants Database
• Purdue University Hort 410 (Vegetable Crops)
• Vascular plant image gallery