Sexual determination in plants

The degree of outcrossing ranges from… 0% 100% hermaphrodites monoecydioecy imperfect perfect

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 Thrum-eyed primrosePin-eyed primrose

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

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

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:

Monoecy Male sterility for c ontrolling 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

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: