Recombination nodules and their role in meiotic recombination

Physical Basis of Heredity 2.  Cell Division (Mitosis and Meiosis)
Significance of meiosis
Reproductive cycles
Stages of meiosis
Synaptonemal complex
Recombination nodules
Comparison of meiosis and mitosis

The synaptonemal complex (SC), described in the preceding section, is though associated with all paired chromosomes at pachytene, its most likely function is perhaps restricted to meiotic synapsis and/or maintenance of the synaptic state. Another class of important structures, associated with paired pachytene chromosomes are 'recombination nodules', which are believed to be involved in meiotic recombination.

A correspondence between meiotic exchange events and the numbers and locations of recombination nodules has been observed in a variety of materials suggesting the possible role of these structures in recombination. In Drosophila females, two types of recombination nodules, spherical (larger in size) and ellipsoidal (smaller in size), have been reported. Association of an ellipsoidal recombination nodule with a synaptonemal complex (SC) is shown in Figure 7.15.
An ellipsoidal recombination nodule associated with paired pachytene chromosomes in Drosophila.
Fig. 7.15. An ellipsoidal recombination nodule associated with paired pachytene chromosomes in Drosophila.

A number of Drosophila mutants, which are defective in recombination, were found to have normal synaptonemal complex, but were found to have changes in the number and morphology of one or both the types of recombination nodules. These observations suggested a positive role of recombination nodules in exchange of chromosome segments during pachytene leading to recombination.

Although data from many organisms firmly establish the correlations between recombination nodules and meiotic recombination (in number/nucleus, number/bivalent arm, distribution on bivalent arms, etc.), their exact role in recombination is not clearly understood. A study of mutants defective for recombination suggested that role of these nodules can not be trivial. These nodules in some way perform the following two distinct roles : (i) they help in choice of number and location of recombination sites and (ii) also help in the recombination itself, by performing enzymatic and supporting functions.

A study of mutants in Drosophila also suggested similar effects on both the ellipsoidal and spherical nodules, so that both of them look related, although the nature of relationship is not clearly known. According to one hypothesis, ellipsoidal nodules may be precursors of spherical nodules, since they appear and disappear earlier than the spherical nodules, although during some developmental stages (a part of pachytene) both are found.