Abstract
Triploid plants are usually highly aborted owing to unbalanced meiotic chromosome segregation, but limited viable gametes can participate in the transition to different ploidy levels. In this study, numerous meiotic abnormalities were found with high frequency in an intersectional allotriploid poplar (Populus alba × P. berolinensis ‘Yinzhong’), including univalents, precocious chromosome migration, lagging chromosomes, chromosome bridges, micronuclei, and precocious cytokinesis, indicating high genetic imbalance in this allotriploid. Some micronuclei trigger mini-spindle formation in metaphase II and participate in cytokinesis to form polyads with microcytes. Unbalanced chromosome segregation and chromosome elimination resulted in the formation of microspores with aneuploid chromosome sets. Fusion of sister nuclei occurs in microsporocytes with precocious cytokinesis, which could form second meiotic division restitution (SDR)-type gametes. However, SDR-type gametes likely contain incomplete chromosome sets due to unbalanced segregation of homologous chromosomes during the first meiotic division in triploids. Misorientation of spindles during the second meiotic division, such as fused and tripolar spindles with low frequency, could result in the formation of first meiotic division restitution (FDR)-type unreduced gametes, which most likely contain three complete chromosome sets. Although ‘Yinzhong’ yields 88.7% stainable pollen grains with wide diameter variation from 23.9 to 61.3 μm, the pollen viability is poor (2.78% ± 0.38). A cross of ‘Yinzhong’ pollen with a diploid female clone produced progeny with extensive segregation of ploidy levels, including 29 diploids, 18 triploids, 4 tetraploids, and 48 aneuploids, suggesting the formation of viable aneuploidy and unreduced pollen in ‘Yinzhong’. Individuals with different chromosome compositions are potential to analyze chromosomal function and to integrate the chromosomal dosage variation into breeding programs of Populus.
Highlights
Polyploidization is an important driving force in plant speciation and evolution [1,2]
‘Yinzhong’ was previously determined to be a triploid by Chen et al [26], the ploidy level of the male clone used in this study was detected to avoid errors
A large number of meiotic abnormalities were recorded in this allotriploid ‘Yinzhong’ (Table 2), showing unbalanced chromosome segregation
Summary
Polyploidization is an important driving force in plant speciation and evolution [1,2]. During the evolution process of plants, crosses between different ploidy levels contribute to chromosomal introgression and genetic diversity [3,4,5]. Polyploids with odd numbers of chromosome sets, especially triploids, are usually unstable, which can either be sterile to be aborted in the evolution process due to their reproductive barrier, or contribute to production of polyploid gametes, depending on the species [8,9,10]. Meiotic nuclear division restitution in odd polyploids can produce unreduced viable gametes, the production of unreduced gametes is low frequency [17]. The production of viable aneuploidy and unreduced gametes suggests that polyploids with odd ploidy levels have value in acting as a bridge for the induction of chromosomal number variation [17,18,19]
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