Mechanisms, Machinery, and Dynamics of Chromosome Segregation in Zea mays

Abstract

Zea mays (maize) is both an agronomically important crop and a powerful genetic model system with an extensive molecular toolkit and genomic resources. With these tools, maize is an optimal system for cytogenetic study, particularly in the investigation of chromosome segregation. Here, we review the advances made in maize chromosome segregation, specifically in the regulation and dynamic assembly of the mitotic and meiotic spindle, the inheritance and mechanisms of the abnormal chromosome variant Ab10, the regulation of chromosome-spindle interactions via the spindle assembly checkpoint, and the function of kinetochore proteins that bridge chromosomes and spindles. In this review, we discuss these processes in a species-specific context including features that are both conserved and unique to Z. mays. Additionally, we highlight new protein structure prediction tools and make use of these tools to identify several novel kinetochore and spindle assembly checkpoint proteins in Z. mays.

Keywords: Zea mays; chromosome; kinetochore; meiosis; meiotic drive; mitosis; spindle; spindle assembly checkpoint.

Figure 1

Chromosome segregation machinery. The segregation machinery that separates chromosomes in mitosis and meiosis is the spindle, a microtubule-based structure. (A) Spindle microtubules attach to kinetochores, multi-protein structures that assemble on centromeres. The dashed box shows a zoomed in image of a kinetochore and includes the proteins identified in Z. mays (kinetochore proteins NDC80, MIS12, KNL1, CENPC and CENH3, and spindle checkpoint proteins MAD2, BUB1, and BUB3 that localize on the outer kinetochore). Replicated chromosomes contain two sister chromatids held together by cohesin until cleavage in mitotic anaphase or meiotic anaphase II. (B) In meiosis I, the spindle segregates homologous chromosomes, and in meiosis II and mitosis, the spindle segregates sister chromatids after cohesin is degraded. The dashed line indicates the plane of cell division.

Figure 2

Zea mays spindles. Live images of Z. mays spindles can be acquired due to the development of fluorescently tagged tubulin. In all images, B-tubulin was tagged with a florescent protein to facilitate imaging. (A) Maize mitotic spindle; microtubules are shown in white (image courtesy of Carolyn G. Rasmussen). (B) Maize meiosis I spindle; microtubules are shown in white. (C) Maize meiosis II spindles, microtubules are shown in white. There are two cells, each containing a meiotic spindle. (D) Time course of meiosis I spindle assembly (0–20 min), alignment of chromosomes on the metaphase I spindle (30 min), and anaphase I segregation of chromosomes (35–45 min). Microtubules are shown in green and chromosomes (labelled with SYTO12 DNA stain) are shown in pink. At time point 0 min, the nuclear envelope is still intact and microtubules can be seen encircling the nuclear membrane. At time point 45 min, the spindle has disassembled and the remaining microtubule structure between the separated chromosomes is the phragmoplast.

Figure 3

Abnormal chromosome 10 (Ab10) structure and function in maize meiotic drive. (A) Ab10 differs from N10 due to the presence of heterochromatic repetitive DNA sequences, TR-1 (shown in red), and knob180 (shown in green). Ab10 also contains genes that code for the TRKIN and KINDR kinesins that associate with TR-1 and knob180 sequences, respectively. (B) The kinesins interact with the TR-1 and knob180 sequence to create neocentromere function capable of binding microtubules and pulling Ab10 chromosomes preferentially toward the developing egg cell. This neocentromere activity is functional before kinetochores are operational on centromeres, thus giving Ab10 chromosomes an advantage due to early interaction with spindle microtubules. (C) There are three distinct Ab10 types (type I, II, III) that can be cytologically distinguished from N10 due to the presence of TR-1 and knob180 sequences. K10L2 is another chromosome 10 variant that is unique from both N10 and Ab10. All variants show differing rates of meiotic drive, likely due to the difference the amount and location of knob DNA sequences.