Cytological and morphology characteristics of natural microsporogenesis within Camellia oleifera

Abstract

Camellia oleifera is believed to exhibit a complex intraspecific polyploidy phenomenon. Abnormal microsporogenesis can promote the formation of unreduced gametes in plants and lead to sexual polyploidy, so it is hypothesized that improper meiosis probably results in the formation of natural polyploidy in Camellia oleifera. In this study, based on the cytological observation of meiosis in pollen mother cells (PMCs), we found natural 2n pollen for the first time in Camellia oleifera, which may lead to the formation of natural polyploids by sexual polyploidization. Additionally, abnormal cytological behaviour during meiosis, including univalent chromosomes, extraequatorial chromosomes, early segregation, laggard chromosomes, chromosome stickiness, asynchronous meiosis and deviant cytokinesis (monad, dyads, triads), was observed, which could be the cause of 2n pollen formation. Moreover, we confirmed a relationship among the length-width ratio of flower buds, stylet length and microsporogenesis. This result suggested that we can immediately determine the microsporogenesis stages by phenotypic characteristics, which may be applicable to breeding advanced germplasm in Camellia oleifera.

Supplementary information: The online version contains supplementary material available at 10.1007/s12298-021-01002-5.

Keywords: Camellia oleifera; Meiosis; Microsporogenesis; Unreduced pollens.

Fig. 1

Flower bud growth characteristics of one flowering branch. A The branch included larger small branches numbered with lowercase letters according to their locations. Each sub-flowering branch was numbered with increasing Arabic numerals from the bottom to the top on the small branch. B The external morphological characteristics of stamens, including long stamens and short stamens shown in a single flower bud

Fig. 2

The discovery and occurrence frequency of natural unreduced pollen of C. oleifera. a Morphological observation and occurrence frequency of natural unreduced pollen. The arrow shows the morphology of unreduced pollen, ** represents extremely significant differences at the 1% level; (b-c). Ploidy detection of the hybrid parent and offspring by flow cytometry (d). Pollen ploidy detection by flow cytometry; np: normal pollen, up: unreduced pollen; the arrow shows the flow cytometry peak diagram of DNA content for unreduced pollen (e–f). Chromosome preparation of the somatic cells of the hybrid parent and progeny

Fig. 3

The microsporogenesis of PMCs in C. oleifera, scale bar = 10 mm. a Early leptotene; b Late leptotene; c Zygotene; d Pachytene; e Diplotene;f Diakinesis; g Metaphase I; h–i Anaphase I; j Telophase I; k Prophase II; l Metaphase II; m–n Anaphase II; o Telophase II; p Cytokinesis; q Tetrad; r Microspore

Fig. 4

The change in the width to length ratio of flower buds during the meiosis of PMCs in C. oleifera. a Flower bud morphology in Prophase I, Scale bar = 10 mm; b Stage 1. Leptotene to late Leptotene; Stage 2. Pachytene to Diplotene; Stage 3. Diakinesis period; 4. Metaphase I to Anaphase II; Stage 5. Telophase II period; Stage 6. Tetrad period; Stage 7. Microspore period. Scale bar = 10 μm

Fig. 5

Abnormal chromosomal behaviour (arrows) in PMCs during meiosis, Scale bar = 10 μm. Observation of the abnormal formation (arrows) of diploid, triad and tetrad and cell plates after meiosis of the PMCs, Scale bar = 5 μm. a univalent chromosomes; b extraequatorial chromosomes; c early segregation; d laggard chromosomes; e chromosome stickiness; f asynchronous meiosis; g dyad; h-j unbalanced cytokinesis, h triad; j. tetrad; k dyad with abnormal cell plate; l triad with abnormal cell plate; m tetrad with abnormal cell plate