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As discussed earlier, at metaphase in mitosis and meiosis II, sister kinetochores attach to spindle microtubules emanating from opposite spindle poles; the kinetochores are said to be bi-oriented. This is essential for segregation of sister chromatids to different daughter cells. In contrast, at meiosis I metaphase, sister kinetochores attach to spindle microtubules emanating from the same spindle pole; these sister kinetochores are said to be co-oriented (see Figure 19-36). Obviously, attachment of sister kinetochores to the proper microtubules in meiosis I and II is critical for correct meiotic segregation of chromosomes.

Proteins required for meiosis I sister kinetochore co-orientation were first identified in S. cerevisiae. In this organism, a single microtubule attaches to each kinetochore. We now know that a protein complex known as the monopolin complex associates with sister kinetochores during meiosis I and fuses them into a single kinetochore unit, to which one microtubule attaches. In all other organisms, kinetochores attach to multiple microtubules. In these organisms, Rec8-containing cohesins are essential for sister kinetochore co-orientation. These meiosis-specific cohesins impose a rigid kinetochore structure, restricting the movement of sister kinetochores and thereby favoring their attachment to microtubules from the same spindle pole.

Correct attachment of meiosis I chromosomes is mediated by a tension-based mechanism, as it is during mitosis and meiosis II. During meiotic metaphase I, kinetochore-associated microtubules are under tension (even though the co-oriented kinetochores of sister chromatids attach to microtubules coming from the same spindle pole) because chiasmata generated by recombination between homologous chromosomes and the cohesins distal to the chiasmata prevent them from being pulled to the poles (see Figure 19-36). Since kinetochore-microtubule attachments are unstable in the absence of tension (due to Aurora B–mediated phosphorylation), kinetochores that attach to microtubules from the wrong spindle release those microtubules, which enables them to bind microtubules again until attachments are made that generate tension. As in mitosis, once tension is generated, microtubule attachment to the kinetochores is stabilized.