HE mur CMS SYSTEM IN Brassica napus

Peter B.E. McVetty¹, Teresio C. Riungu² and Rachael Scarth¹

¹Dept. of Plant Science, University of Manitoba, Winnipeg, MB, Canada R3T 2N2

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²National Plant Breeding, Research Centre, P.O. Njoro, Kenya

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ABSTRACT

 Many CMS systems, intended to permit the economic production of hybrid B. napus cultivars,  have been created or discovered in B. napus in recent decades,  including the mur CMS system.   It has been difficult to develop a complete mur CMS system in B. napus because of the consistent disappearance of male sterility in advancing backcrosses of D. muralis X B. napus intergeneric crosses.  It was first thought that male sterility observed in these intergeneric hybrids was due to aneuploidy. However, it was later determined that the male sterility disappearance in advancing backcrosses of D. muralis X B. napus intergeneric crosses occurred because the B. napus genotypes used in these crosses carried one or more restorer genes for the mur male sterility inducing cytoplasm.   Fortunately, a single maintainer genotype was discovered in a semi-winter B. napus  rapeseed of Asian origin.  This permitted the initial development of a semi-winter rapeseed male sterile line (A-line) in the mur cytoplasm, and its complementary maintainer line (B-line) in the nap cytoplasm.   This semi-winter rapeseed mur CMS  A-line and B-line pair was used in this study to i) investigate the frequency of occurrence of  mur CMS system maintainer genotypes in summer B. napus cultivars, ii) investigate the inheritance of  mur CMS system maintenance and restoration in summer B. napus, iii) investigate the temperature stability of derived summer B. napus mur CMS A-lines, iv) develop mur CMS A-line and B-lines pairs in summer B. napus and v) investigate the biological cost of the mur CMS system on the agronomic performance and seed quality characteristics of mur CMS system summer B. napus hybrids.

KEYWORDS

pollination control, hybrids, heterosis, biological cost

INTRODUCTION

Many CMS systems, intended to permit the economic production of hybrid B. napus cultivars,  have been created or discovered in the Brassicaceae  in recent decades, including the mur CMS system.  Hinata and Konno (1979) were the first to report that the D. muralis  cytoplasm induced male sterility in  B. rapa.  Shiga (1980) reported the first instance of D. muralis cytoplasm-inducing male sterility in B. napus. In contrast, Pellan-Delorme and Renard (1987) found that crosses of B. rapa with the D. muralis cytoplasm to B. napus genotypes produced only male fertile progeny.    Crosses of D. muralis with the Canadian cultivar Regent followed by six backcrosses of Regent to the non-recurrent parent  produced some sterile plants but a segregation ratio of male sterile to male fertile plants could not be determined (Fan et al. 1985).  It was observed that the male sterile plants occurring in the backcross progeny all carried a single supernumerary chromosome. Fan et al. (1985) therefore concluded that the male sterility was due to aneuploidy.   However, it was later determined that the male sterility disappearance in advancing backcrosses of D. muralis X B. napus intergeneric crosses occurred because the cultivar Regent  carried one or more restorer genes for the mur male sterility inducing cytoplasm (Riungu 1995).  Fortunately, a single maintainer genotype was discovered in a semi-winter B. napus  rapeseed of Asian origin.  This permitted the initial development of a semi-winter rapeseed male sterile line (A-line) in the mur cytoplasm, and its complementary maintainer line (B-line) in the nap cytoplasm.  This semi-winter rapeseed mur CMS  A-line and B-line pair was used in this study to  i) investigate the frequency of occurrence of  mur CMS system maintainer genotypes in summer B. napus cultivars, ii) investigate the inheritance of  mur CMS system maintenance and restoration in summer B. napus, iii) investigate the temperature stability of derived summer B. napus mur CMS A-lines, iv) develop mur CMS A-line and B-lines pairs in summer B. napus and v) investigate the biological cost of the mur CMS system on the agronomic performance and seed quality characteristics of mur CMS system summer B. napus hybrids.  Several papers detailing this research have been submitted or in preparation, therefore,  this paper intentionally presents a preliminary overview of these results only.

METHODS AND MATERIALS

i) To investigate the frequency of occurrence of  mur CMS system maintainer genotypes in summer B. napus cultivars.

Hand crosses of over 100 spring habit B. napus genotypes from diverse origins were crossed to a semi-winter habit mur CMS A-line.  The mur CMS A-line was also crossed to its corresponding B-line.

A minimum of 50 plants for each F₁ were evaluated for male fertility/male sterility.

ii) To investigate the inheritance of  mur CMS system maintenance and restoration in summer B. napus.

The F₁'s of eight of the above crosses of diverse origin were grown in a growth room and selfed to generate F₂ seed for each cross.  A minimum of 500 F₂ plants per cross were scored as either male fertile or male sterile in the field.

iii) To investigate the temperature stability of derived summer B. napus mur CMS A-lines.

Three BC6 mur CMS A-lines of diverse origins,  the semi-winter habit  mur CMS A-line and the best pol CMS A-line developed at the University of Manitoba were treated for seven days in the bud stage, to temperatures of 22/16EC or 30/24EC in separate growth cabinets.  These treated plants were then put in a greenhouse and grown to flowering to enable the assessment of male sterility index (MSI) (Burns et al. 1991).

iv) To develop mur CMS A-line and B-lines pairs in summer B. napus.

Three BC6 mur CMS A-lines of diverse origins were created using the available semi-winter mur CMS A-line and B-line using conventional backcrossing techniques combined with a testcross to identify F₂ mur CMS maintainer genotypes in each backcross cycle.

v) To investigate the biological cost of the mur CMS system on the agronomic performance and seed quality characteristics of mur CMS system summer B. napus hybrids.

Six hybrids in the mur CMS cytoplasm, created using the mur CMS A-lines developed above, and six genetically identical hybrids, created using the nap cytoplasm mur CMS B-lines created above, along with the four open pollinated male parents used in the crosses were grown in replicated yield trials in four environments and assessed for agronomic and seed quality traits.  The mur CMS hybrids were compared as a group to the nap CMS hybrids to determine if there were any significant differences in agronomic or quality characteristics performance between the two groups.

RESULTS AND DISCUSSION

i) Frequency of occurrence of  mur CMS system maintainer genotypes in summer B. napus cultivars.

All F₁ plants in all crosses of over 100 spring habit B. napus genotypes from diverse origins, crossed to a semi-winter habit mur CMS A-line were completely male fertile (i.e. restored).  In contrast, the F₁ plants from the mur CMS A-line crossed to its corresponding B-line were completely male sterile  (i.e. maintained).  The frequency of mur CMS maintainer genotypes in this genetically diverse sample of spring habit  B. napus genotypes was therefore zero.  This suggests that the frequency of naturally occurring mur CMS maintainer genotypes in spring habit B. napus is very low, perhaps even zero. 

ii) Inheritance of  mur CMS system maintenance and restoration in summer B. napus.

A minimum of 500 F₂ plants per cross from each if eight crosses of genetically diverse cultivars crossed to the mur CMS A-line were grown in the field and were scored as either male fertile (i.e. restored) or male sterile (i.e. maintained).  All F₂ populations segregated for male fertile and male sterile plants and these segregation ratios were used to determine the number of restorer genes present in each male parent cultivar.  One to three restorer genes were found to be present in the samples of diverse spring habit B. napus cultivars tested.  Some cultivars were heterogeneous, with different portions of the open pollinated cultivar population containing different numbers of restorer genes.   All spring habit B. napus cultivars tested contained at least one restorer gene and many contained two or three restorer genes.

iii) Temperature stability of derived summer B. napus mur CMS A-lines.

The male sterility index (MSI) values for the control treatment for the three BC6 mur CMS A-lines of diverse origins,  the semi-winter habit mur CMS A-line and the best pol CMS A-line developed at the University of Manitoba were very low on the 0 to 6 scale (i.e. no pollen production on any anther).  The MSI values for the heat treatment for the three BC6 mur CMS A-lines of diverse origins,  the semi-winter habit  mur CMS A-line were also very low on the 0 to 6 scale (i.e. no pollen production on any anther) while the MSI increased to approximately 3 for the best pol CMS A-line developed at the University of Manitoba.  These results indicate that the mur CMS A-lines evaluated in this study had temperature stable male sterility to temperatures of 30/24EC in the bud stage.  The mur CMS A-lies  were much more temperature stable than the best pol CMS A-lines available at the University of Manitoba.

iv) Mur CMS A-line and B-lines pair development in summer B. napus.

Three BC6 mur CMS A-lines and B-lines of diverse origins were created using conventional breeding methods.  These mur CMS A-lines were agronomically similar to their respective B-lines and all had excellent male sterility.  They were not canola quality, however, and further breeding work to develop canola quality mur CMS A-lines is ongoing.

v) Biological cost of the mur CMS system on the agronomic performance and seed quality characteristics of mur CMS system summer B. napus hybrids.

The eight mur CMS hybrids, compared to the genetically identical nap CMS hybrids were not significantly different for 1) days to emergence, 2) vigor, 3) days to first flowering, 4) height or 5) lodging. The eight mur CMS hybrids, compared to the genetically identical nap CMS hybrids were, however,  significantly different for 6) seed yield, 7) total dry matter production, 8) harvest index, 9) oil content and 10) protein content.  These differences, although statistically significant, were small in practical terms.  The minimal biological costs associated with the mur CMS system suggest that it should be developed into a fully functional CMS system for use in hybrid canola/rapeseed cultivar development programs.

REFERENCES

Burns, D.R., R. Scarth and P.B.E. McVetty (1991).  Temperature and genotypes effects on the expression of pol cytoplasmic male sterility in summer rape.  Canadian Journal of Plant Science. 71:655-661.

Fan, Z., W. Tai and B.R. Stefansson (1985).  Male sterility in Brassica napus associated with an extra chromosome.  Canadian Journal of Genetics and  Cytology 27:467-471. 

Hinata, K. and N. Konno (1979).  Studies on a male sterile strain having Brassica campestris nucleus and Diplotaxis muralis cytoplasm.  I. Breeding and some characteristics of this strain. Japanese Journal of Breeding. 29:305-311.

Pellan-Delorme, R., and M. Renard (1987).  Identification of maintenance genes in Brassica napus L. for the male sterility inducing cytoplasm of Diplotaxis muralis L.  Zeitschrift Pflanzenzuchtung. Berlin.  99:89-97.

Riungu, T. C. (1995).  Development and Evaluation of the Diplotaxis muralis L. Cytoplasmic Male Sterility System in Summer Rape (Brassica napus L.).  Ph.D. thesis, University of Manitoba, Winnipeg, Manitoba, Canada.

Shiga, T. (1980).  Male sterility and cytoplasmic differentiation.  In “Brassica Crops and Wild Allies, Biology and Breeding”.  (S. Tsunoda, K. Hinata and C. Gomez-Campo eds.). pp 205-257.  Japanese Science Society Press, Tokyo, Japan.