Planta <1994)193:115-122 Planta Springer-Verlag 1994 Populations of plastids and mitochondria during male reproductive cell maturation in Nicotiana tabacum L.: A cytological basis for occasional biparental inheritance Hong-Shi Yu, Scott D. Russell Department of Botany and Microbiology,Universityof Oklahoma, Norman, OK 73019, USA Received: 16 August 1993 / Accepted: 21 September 1993 Abstract. The dynamics of plastid and mitochondrial populations in male reproductive cells of tobacco (Nico- tiana tabacum L.) were examined during development us- ing serial ultrathin sections and transmission electron mi- croscopy to reconstruct 58 generative cells and 31 sperm cells at selected stages of maturation from generative cell formation through gametic fusion. The first haploid mi- tosis resulted in incomplete exclusion of plastids provid- ing an average of 2.81 plastids and 82.7 mitochondria for each newly formed generative cell. During generative-cell maturation, plastid content decreased to an average of 0.48 plastids/generative cell at anthesis owing to au- tophagy of organelles. Plastids were present in low fre- quency within generative and sperm cells in the pollen tube and appeared to be transmitted, according to obser- vations immediately prior to fertilization. This forms a cytological basis for genetic reports of occasional bi- parental plastid inheritance. In contrast, mitochondria were transmitted in larger numbers, and approximately 80 mitochondria per generative cell or sperm cell pair were retained throughout development. This provides a potentially stable source for the transmission of male mi- tochondrial DNA, if present at fertilization. Key words: Autophagy- Cytoplasmic inheritance Gen- erative cell - Nicotiana (reproductive cells) - Plastid diminution Sperm cell Introduction The inheritance of plastids in flowering plants has been categorized into three types: maternal, paternal and bi- parental. Uniparental, maternal inheritance is the preva- lent pattern, found in the majority of angiosperms. Bi- parental inheritance occurs in a minority of higher Abbreviations: GC - generativecell; SC = sperm cell Correspondence to: S.D. Russell; FAX: 1 (405) 325 7619 plants, and exclusively paternal inheritance has just been reported recently in a few taxa (for reviews, see Kirk and Tilney-Bassett 1978; Sears 1980; Smith 1988; Russell 1992). A number of cytological mechanisms have been pro- posed for uniparental maternal inheritance of plastids, including: (i) exclusion of plastids during the first and/or second post-meiotic mitosis (Hagemann 1981), (ii) degen- eration or alteration of plastids during the maturation of the generative cell (GC) and sperm cell (SC), and (iii) elimination of plastids during the process of fertilization (Hagemann and Schr6der 1989). For species to display biparental plastid inheritance, both male and female gametes must contain plastids that are transmitted into the next generation. In the case of exclusively paternal plastid inheritance (Medicago, Zhu et al. 1993; Daucus, Hause 1991), maternal plastids are distributed toward the micropylar end of the egg cell, apparently excluding ma- ternal plastids from the lineage of the embryo proper at the first division of the zygote. Small proportions of hybrid offspring have been re- ported recently in plants previously thought to exhibit exclusively maternal plastid inheritance (Cornu and Dulieu 1988; Dulieu et al. 1990; see also Table 9.3 in Smith 1988). Smith (1988) therefore distinguishes species with biparental plastid inheritance into those species with "regular" occurrence of paternal plastids (>95% with paternal plastid expression) from those with "occa- sional" transmission (< 5% with paternal plastid expres- sion). Medgyesy et al. (1986) revealed a very low frequency of paternal plastid expression in the progeny of Nicotiana tabacum (0.07% for interspecific crosses, and 2.5% for intraspecific crosses) thereby indicating that tobacco con- forms to the latter pattern. In the current study, the or- ganelle content of generative and sperm cells was exam- ined during development to determine the dynamics of plastid and mitochondrial populations and to evaluate whether there exist in the male gamete transmissible plas- tids and mitochondria which may be inherited in the em- bryo.