1. Sexual determination in plants

2. The degree of outcrossing ranges from…
perfect
imperfect
hermaphrodites
monoecy
dioecy 0%
100%

3. Perfect flowers: 90% of angiosperms are hermaphrodites.
The ancestral and basal condition is hermaphroditic

4. The benefits of outcrossing often outweigh the drawbacks
Avoids inbreeding depression
Promotes heterozygosity
Promotes genetic variability
Drawbacks
Environmental conditions may limit pollen flow
"Cost" of having two parents, only one of which bears seed

5. Even with perfect flowers, there are ways of encouraging, or even requiring, outcrossing
Structural differences - Heterostyly
Pin-eyed primrose
Thrum-eyed primrose

7. Even with perfect flowers, there are ways of encouraging, or even requiring, outcrossing
Male sterility

8. Imperfect flowers – encouraging and/or obliging outcrossing
The basis of sex determination is selective abortion of flower organs: The "basal condition" is hermaphroditic; different species differentiate at different times

9. Monoecy 
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 
Source: passel.unl.edu

10. Monoecy 

11. Monoecy Incompatibility in Hazelnut
Self incompatibility can reinforce separation of sexes on the same plant
SI: Example - Corylus spp.  
Incompatibility in Hazelnut
One S-locus, 30 alleles
Co-dominance in Stigmas
Dominance or Co-dominance in Pollen
Fluorescence Microscopy
If the same allele is expressed by the stigma
and the pollen, the cross is incompatible
Source: S. Mehlenbacher, OSU

12. Incompatibility Testing using Fluorescence Microscopy
Compatible
Excellent germination
Long parallel tubes
Incompatible
Poor germination
Short tubes, bulbs
Source: S. Mehlenbacher, OSU

13. Monoecy
Male sterility can reinforce separation of sexes on the same plant
MS: Nuclear or cytoplasmic
Example of CMS - Zea mays “T” cytoplasm
Mitochondrial / nuclear gene interaction
Pleiotropic effects: sterility and disease susceptibility
Source:

14. Monoecy
Male sterility for controlling GMO gene flow in Zea mays

15. Dioecy
Distinguishing feature: sex chromosomes. Parallels to XY (mammals) and X:A ratio (Drosophila)
Humulus lupulus
Plant achieves a vegetative to reproductive transition 
Selective elimination of organs in staminate and pistillate flowers
Sex-determining genes concentrated on X and Y chromosomes
XX = female; XY = male

16. Dioecy

17. Dioecy Evolution of sex chromosomes from autosomes
Accumulation of sex-determining genes on a single chromosome with no homolog prevent recombination between sex-determining genes
Create ~ equal numbers of male and female offspring theoretically leads to degeneracy of Y – except for “maleness” genes. But the Y chromosome is not inert
Y chromosomes are in a permanent haploid state

18. Dioecy
Example: Asparagus officinalis: Using doubled haploids to produce YY “super males”
Males are XY or YY
Males have increased vigor and therefore optimum for production
XX x YY = all male (XY)
Source: