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Sexual systems in angiosperms 

Perfect flowers
Both male and female parts are present in the same flower.  Individuals having both male and female reproductive parts are called hermaphorditic.
Some examples are shown below.


Aesculus hippocastanum (horse chestnut) - The style is noticeably thicker than the filaments suspending the anthers.

Aesculus pavia (red buckeye) - In the flower at the top, the anthers are exerted first.  In the flowers at the bottom, the stigma has now emerged from the petals.

Tilia americana (basswood) - The conical style projects above the numerous athers.

Magnolia virginiana (sweetbay magnolia) - Multiple stigmas project above rows of anthers in this flower that has not yet opened.  

Monoecious flowers
Separate male and female flowers are present on the same individual plant.
Example: Betula species (birch)


The green female inflorescence projects upward on the left side of the branch.  The male catkin hangs below the branch on the right.  

Individual flowers with their two stigmas can be seen in this closeup of the female inflorescence.  

The individual anthers can be seen projecting beyond the scales of the catkin.

Dioecious flowers
Male and female flowers are found on separate individual plants
Example: Maclura pomifera (Osage orange)


The male flowers form a loose round head.

Female flowers form a dense round head which develops into an unusual green fruit. 

Example: Silene latifolia ssp. alba (bladder campion)


The anthers project from the corolla tube in the male flowers.

Although female flowers are superficially similar, stigmas project out of the corolla tube. 

 

Other variations: gynodioecy
Part of the population is hermaphroditic, part of the population is female.
Example:
Silene vulgaris (bladder campion)

Hermaphroditic individuals can reproduce through both seeds and pollen. However, a small fraction of plant species display a phenomenon known as gynodioecy. In gynodioecious species, both hermaphroditic and functionally female individuals can be found in the same population. In some species females can approach 50% of the population. How can females persist evolutionarily, given that on average they have lost 50% of their reproductive potential when compared to hermaphrodites, due to the loss of pollen? 

In most gynodioecious plant species the female phenotype is due to a mutation in the mitochondrial genome know as a cytoplasmic male sterility (CMS) factor. Unlike nuclear genes, mitochondrial genes are usually inherited only through the mother. That means that they are passed on to the next generation through seed production, but not through pollen. Therefore, if a CMS mutation increases seed production it will be favored evolutionarily, even through it cuts off pollen production. Why should CMS factors increase seed production? Studies have shown that in gynodioecious species, females produce more seeds than hermaphrodites because energy that would have gone to producing pollen can now go to producing seeds. Further, females often produce more vigorous seeds. One disadvantage to being a hermaphrodite is that self-fertilization is possible. This inbreeding can lead to a loss of vigor in the offspring (inbreeding depression). Since females can not self-fertilize, their offspring are much less likely to suffer from inbreeding depression. 


Hermaphrodite in initial male phase.  Only the anthers are exerted outside the flower in this stage.

Hermaphrodite in later female phase. Both anthers and stigmas are exerted.  Note the pollen-heavy anthers.  

Female flowers.  Note the the vestigial anthers inside.  

Dr. David McCauley of the Vanderbilt University Department of Biological Sciences studies gynodioecy in Silene vulgaris. Populations of S. vulgaris range from having more than 50% female to having nearly all hermaphrodites. Dr. McCauley is studying the selective factors that regulate the frequency of females within populations (why are they so much more common in some areas than others?), and the genetic basis of sex determination. These studies include field work designed to study the reproductive success of females and hermaphrodites in natural populations in the mountains of Virginia, greenhouse genetic crosses, and the use of molecular markers designed to locate the genes responsible for sex expression.

Images copyright 2002 Steve Baskauf - Terms of use