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Sexual determination in
plants
http://www2.dupont.com/home/en-us/index.html
The degree of outcrossing ranges from…
imperfect
perfect
hermaphrodites
0%
monoecy
dioecy
100%
Perfect flowers: 90% of angiosperms are
hermaphrodites.
The ancestral and basal condition is hermaphroditic
The benefits of outcrossing often outweigh the
drawbacks
Benefits
• 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
Even with perfect flowers, there are ways of
encouraging, or even requiring, outcrossing
Structural differences - Heterostyly
Pin-eyed primrose
Thrum-eyed primrose
Even with perfect flowers, there are ways of
encouraging, or even requiring, outcrossing
Male sterility
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
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
Monoecy
Monoecy
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
Incompatibility Testing using Fluorescence
Microscopy
Compatible
• Excellent germination
• Long parallel tubes
Incompatible
• Poor germination
• Short tubes, bulbs
Source: S. Mehlenbacher, OSU
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:http://metos.at/tiki/img/wiki_up/image/hturcicum1.jpg
Monoecy
Male sterility for controlling GMO gene flow in Zea mays
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
Dioecy
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
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: http://aesop.rutgers.edu/~asparagus/program/home.html