Fig. 1 - Tomato pachytene (meiotic) chromosomes (= bivalents).  Each bivalent is composed of two synapsed homologous chromosomes.   (A) The set at the top has been stained with CPD.  GC-rich areas on chromosomes appear pink.  The nucleolus organizer region (NOR, extended pink region at top) is in contact with the nucleolus (quasi-spherical structure at top).  (B) Tomato pachytene chromosomes hybridized with TGRI, a repetitive DNA sequence located near most telomeres and at a few interstitial chromosomal sites (yellow foci).  Note the nucleolus.

Fig. 2 - Sorghum pachytene (meiotic) chromosomes.  Fluorescence in situ hybridization was used to visualize the centromeres of the bivalents.  The probe is pSau3A10, a grass-specific centromeric sequence.

Fig. 3 - Set of tomato pachytene (meiotic) chromosomes stained with DAPI.  The nucleolus appears as a spherical structure.  Heterochromatin stains more intensely than euchromatin.  Note the bright-staining, heterochromatic telomeres at the termini of each chromosome.

Fig. 4 - CPD (combined propidium iodide and DAPI staining) of pachytene (meiotic) chromosomes.  From upper left counterclockwise: (A) Tomato chromosomes stained with DAPI only.  (B) Tomato, no RNase treatment.  (C) Tomato, RNase treatment prior to CPD staining.  (D) Maize.  Note the pink color of the NOR.  (E) Maize.  Note heterochromatic knobs, pink NOR, and red nucleolus.  (F) Potato.  Two NORs (red regions) are visible in the chromosome set of this recent tetraploid.

Fig. 5 - Two synaptonemal complexes (SCs) from tomato.  SCs are proteinaceous structures that join homologous chromosomes along their entire length during pachynema of meiosis.  It is within the framework of the SC that recombination takes place.  Each SC has a kinetochore ('ball-like' structure) at  its centromere.  Recombination nodules (RNs), ellipsoidal structures found on the central regions of SCs, mark the sites of crossover events (see inset).  Hence RNs are believed to be responsible for mediating crossing over.

Fig. 6 - Top: Set of tomato synaptonemal complexes (SCs).  Chromatin 'sheaths' are visible around each SC.  Bottom: Two tomato SCs.  The chromatin has been stripped from the SCs allowing the details of the SC to be observed.  Each SC has a kinetochore ('ball-like' structure) at  its centromere.  Recombination nodules (RNs), ellipsoidal structures found on the central regions of SCs, mark the sites of crossover events (see inset).  Hence RNs are believed to be responsible for mediating crossing over.