Epitope tag-specific differences in the detection of COSA-1 marked crossover sites in C. elegans spermatocytes

Abstract

Nascent crossover sites in C. elegans meiocytes can be cytologically detected using epitope-tagged versions of the pro-crossover protein COSA-1. In spermatocytes, differences exist between cytologically-detected and genetically-detected double crossover rates. Here, we examine nascent crossovers using both GFP- and OLLAS-tagged COSA-1. Similar to previous work, we find that most late pachytene spermatocytes display 5 COSA-1 foci, indicating one crossover per autosome bivalent. However, we detected more nuclei with >5 COSA-1 foci using OLLAS::COSA-1, reflecting some bivalents having 2 COSA-1 foci. These results demonstrate tag-specific differences in the detection of COSA-1 marked nascent crossovers in spermatocytes.

Figure 1. COSA-1 foci in late pachytene spermatocytes.

(A) Diagram of the C. elegans male germ line. Arrows indicate direction of meiotic progression. For this study, we subdivided pachytene into three equally sized regions based on the total length of pachytene: early (first third), mid (middle third), and late (last third) (See Methods). (B) Representative images of late pachytene nuclei with 5, 6, or 7 OLLAS::COSA-1 foci (yellow). (C) Bar graph indicating the percentages of late pachytene nuclei with ≤3, 4, 5, 6, or 7 COSA-1 foci detected in strains expressing either OLLAS::COSA-1 or GFP::COSA-1. The numbers within each bar indicate the numbers of nuclei in each category from a total of 8 germ lines for OLLAS::COSA-1 and 7 germ lines for GFP::COSA-1. (D) Representative SIM images depicting spermatocyte chromosome spreads from late pachytene nuclei with 5 or 6 OLLAS::COSA-1 foci (yellow), HTP-3 (red, chromosome axis), and SYP-1 (blue, synaptonemal complex). White arrowheads indicate the unsynapsed partnerless X chromosome. The dashed line in the first column traces the path of a single synapsed chromosome pair with either one COSA-1 focus (top) or two COSA-1 foci (bottom). The chromosome pair with two COSA-1 foci is outlined in the inset shown at the right.