The male reproductive organs are mal-developed or aborted so that no viable pollens will be formed. This condition is known as male sterility (M.S) and may be inherited due to genetic, cytoplasmic or interaction of both genetic and cytoplasmic causes.
History of Male Sterility
- The first documentation of male sterility was done by Joseph Gottlieb Kolreuter, who observed anther abortion within species and specific hybrids.
- Genic male sterility has been reported in cabbage (Rundfeldt,1960), cauliflower (Nieuwhof 1961).
- Male sterility systems have been also developed through genetic engineering (Williams et al. 1997) and protoplast fusion (Pelletier et al.1995).
- Male sterility were artificially induced through mutagenesis (Kaul 1988).
Features of male sterility
- Prevents self pollination, encourages cross pollination.
- Paves the way to heterozygosity.
- Female gametes are functional.
Phenotypic expression of male sterility
- Absence of male sex organ.
- Lack of normal anther sac.
- Inability of the pollen to mature.
- Inability to develop normal pollen.
Male sterility more prevalent than female sterility
(The Answer to Why)
- Less protected: Male sporophyte and gametophyte are more prone to damage from the environment than ovule and embryo sac.
- Easy to detect M.S: Because a large number of pollen for study is viable.
- Easy to assay M.S: Staining technique (carmine, lactophenol or iodine); female sterility requires crossing.
- Propagation potential: M.S has propagation potential in nature (can still set seed) and is important for crop breeding, female sterility does not.
Reasons of Male sterility
- Absence or malformation of male organs (stamens) in bisexual plants or no male flowers in dioecious plants.
- Failure to produce normal pollen due to a breakdown at some stages in pollen
- Abnormal microsporogenesis – deformed or inviable pollen.
- Abnormal pollen maturation; inability to germinate on compatible stigma.
- Pollen may be formed but fail to mature or dehisce due to aberrant structure of anthers.
-Non dehiscent stamens in tomato (Solanum lycopersicum) and grape (Vitis vinifera).
-Absence of anther pore in tomato variety “John Baer”.
-Anther wall too hard to rupture in some cloves (Syzygium aromaticum).
- In dioecious species, a differential survival of female plants or a reversion of sex organs in male plants.
- Barriers other than incompatibility preventing pollen from reaching ovule.
Mechanism of male sterility
Expression of male sterility trait is associated with –
√ Morphological changes
√ Histological changes
√ Cytological changes
√ Biochemical changes
√ Molecular changes
- Complete absence of male reproductive organs.
- Male sterile flowers are commonly smaller in size in comparison to the fertile.
- The size of stamens is generally reduced.
- Role of tapetum (layer that supplies nutrition to developing anthers)
- Premature breakdown of tapetum
- Abnormal development of tapetum in male sterile plants first reported by (Monosmith 1926)
- Role of callase (enzyme that separates the microspores from tetrad)
- Early or delayed callase activities have been found to be associated with male sterility.
- Role of esterase
- Decreased activity of esterase in male sterile plants has been observed in tomato (Bhadula and Sawhney 1987) & in radish (Zhou and Zhang 1994).
- Role of PGR’s (Plant Growth Regulators)
- Reduced level of cytokinins and increased level of abscisic acid associated with GMS (Genetic Male Sterility) and CMS (Cytoplasmic Male Sterility) plants.
In both GMS & CMS systems, male sterility is the consequence of breakdown of tightly regulated pollen development and fertilization processes at any of the pre- or post-meiotic stages i.e
- During the formation of tetrad
- During the release of the tetrad
- At the vacuolated microspore stage or pollen dehiscence stage.
- Reduced level of proline, leucine, isoleucine, phenylalanine and valine.
- Increased level of asparagine, glycine, arginine, aspartic acids.
- Male sterile anthers contain lower protein content and fewer polypeptide bonds.
- Mis-timing of callase activity.
- Decreased activity of esterase and amylases.
Molecular changes in CMS
- CMS resulted from an inability of the mitochondria to meet the energy demands of male gametogenesis.
- Male organ shown lower levels of ATP production and high levels of alternative oxidase activity.
- Mitochondria play a central role in premature programmed cell death.
Types of male sterility
- Genetic controlled male sterility: Governed by nuclear genes.
- Cytoplasmic controlled male sterility: Governed by cytoplasmic factors.
- Cytoplasmic-Genetic controlled Male sterility: Governed by both cytoplasmic and nuclear genes.
Significance of male sterility in plant breeding
- Male sterility is a primary tool to avoid emasculation in hybridization.
- Hybrid production requires a female plant in which no viable pollens are borne. Insufficient emasculation may produce some self fertile progenies.
- GMS is being exploited (Eg. in USA-Castor, in India-Arhar).
- CMS/CGMS are routinely used in Hybrid production in corn, sorghum, sunflower and sugarbeet, ornamental plants.
- Saves lot of time, money and labor.
Male sterility system in Rice hybrid seed production
- Male sterility: A condition in which the pollen grain is non-viable or cannot germinate and fertilize normally to set seeds.
- Male sterility systems (genetic and non -genetic).
a) Cytoplasmic genetic male sterility (CMS): Male sterility is controlled by the interaction of a genetic factor (S) present in the cytoplasm and nuclear gene (s).
b) Environmental-sensitive genic male sterility (EGMS)
c) Chemically induced male sterility: Male sterility is induced by some chemicals (gametocides).
Male sterility in Maize hybrid seed production.
Different ways of inducing male sterility in maize-
- Manual /mechanical emasculation (detasseling)
- Cytoplasmic male sterility
- Cytoplasmic genetic male sterility
- Genetic male sterility
References & Other Links
Slides from Rita Sara Borna Maam, Assistant Professor, University of Dhaka.
Toioba Akter Sagorika, B.S. (Hons), Department of Botany, University of Dhaka