Synaptonemal complex – polyploids

 

The synaptonemal complex (SC) is a proteinaceous structure consisting of two lateral elements and a central element. The SC lies between the homologous chromatids during the homologous pairing stage of mid-prophase I of meiosis in nearly all sexual species.

Most researchers assume that the SC is necessary for proper crossing over and anaphase I segregation of homologues. Techniques developed 20 years ago make it possible to spread the SC onto a plastic film, dispersing the chromatin (i.e., DNA and chromosomal protein) in the process. This allows one to follow the synapsed lateral elements along the entire length in the transmission electron microscope, thereby evaluating in finer detail the pairing of homologous chromosomes (see figure below).

image5.jpg (92455 bytes)

 

 

I am interested in using this SC spreading technique to evaluate pairing in natural polyploids. Polyploids are organisms with three or more full sets of chromosomes (instead of the normal two). Although uncommon in animals, polyploidy is quite common in plants. 50% of all plant species are thought to be of polyploid origin. In most of these species the polyploids are considered "diploidized" whereby the chromosomes associate only as pairs (i.e., bivalents), rather than multiples (trivalents, quadrivalents, etc.) in prophase I of meiosis.

My interest is in autotetraploids that are polyploids with four sets of chromosomes with equal homology among the four. These can be distinguished from allotetraploids in that their chromosomes often associate as quadrivalents in prophase I. This type of pairing is thought to result in reduced fertility relative to alloploid, bivalent pairing. And considered less adaptive evolutionarily than alloploids. Fifteen years ago, I was involved in the development of a model using meiotic data to identify autopolyploids and distinguish them from alloploids. We were not able to analyze SC synapsis at the time and were only able to hypothesize events occurring at this stage in autotetraploids. I would like to apply the SC spreading technique to autotetraploids growing in southeastern Louisiana (i.e., Tradescantia hirsutiflora) to further test the hypotheses upon which our autoploid model is based.

 

Synaptonemal complex – supernumerary chromosomes

Supernumerary chromosomes (a.k.a. B-chromosomes) are extra chromosomes present in many species of plants and animals that do not appear to perform any essential genetic function. Their numbers vary from population to population and from individual to individual. They are usually much smaller than the normal (i.e., A-)chromosomes and are often mitotically and meiotically unstable.

 

In order to better understand the nature of supernumerary chromosomes and their biological as well as evolutionary significance, I would like to investigate SC formation involving the supernumerary chromosomes and compare it to SC formation in normal chromosomes. Also, since supernumerary chromosomes are thought to originate from normal chromosomes, it would be of interest to determine if SC formation occurs between supernumerary chromosomes and A chromosomes.