Inverted meiosis: an alternative way of chromosome segregation for reproduction

Canonical meiosis is characterized by two sequential rounds of nuclear divisions following one round of DNA replication-reductional segregation of homologous chromosomes during the first division and equational segregation of sister chromatids during the second division. Meiosis in an inverted order of two nuclear divisions-inverted meiosis has been observed in several species with holocentromeres as an adaptive strategy to overcome the obstacle in executing a canonical meiosis due to the holocentric chromosome structure. Recent findings of co-existence of inverted and canonical meiosis in two monocentric organisms, human and fission yeast, suggested that inverted meiosis could be common and also lead to the puzzle regarding the mechanistic feasibility for executing two meiosis programs simultaneously. Here, we discuss apparent conflicts for concurrent canonical meiosis and inverted meiosis. Furthermore, we attempt to provide a working model that may be compatible for both forms of meiosis.