cDNA clones from meiotic-specific poly(A)+ RNA in Lilium: Homology with sequences in wheat, rye, and maize

The meiotic transcriptome architecture of plants

Although a number of genes that play key roles during the meiotic process have been characterized in great detail, the whole process of meiosis is still not completely unraveled. To gain insight into the bigger picture, large-scale approaches like RNA-seq and microarray can help to elucidate the transcriptome landscape during plant meiosis, discover co-regulated genes, enriched processes, and highly expressed known and unknown genes which might be important for meiosis. These high-throughput studies are gaining more and more popularity, but their beginnings in plant systems reach back as far as the 1960's. Frequently, whole anthers or post-meiotic pollen were investigated, while less data is available on isolated cells during meiosis, and only few studies addressed the transcriptome of female meiosis. For this review, we compiled meiotic transcriptome studies covering different plant species, and summarized and compared their key findings. Besides pointing to consistent as well as unique discoveries, we finally draw conclusions what can be learned from these studies so far and what should be addressed next.

An early meiosis cDNA clone from wheat

Meiosis occupies only a very short period of the life cycle of higher plants but it is a crucial process ensuring the correct passage and maintenance of genetic information from parent to offspring. A clone (designated pAWJL3) has been isolated from a cDNA library generated from RNA prepared from young wheat florets at early meiosis. The clone was identified through cross-hybridisation to a cDNA clone from maize that, in turn, had been isolated by hybridisation to a Lilium meiosis-specific cDNA clone. The genes encoding the sequence represented in the wheat cDNA clone have been assigned to chromosomes in wheat. The clone, pAWJL3, represents a small family of genes with about 20 members located on the short arms of group 3 and 5 chromosomes. The chromosomal regions harbour genes known to control chromosomal pairing in wheat. DNA prepared from a deletion mutation affecting one of the major genes controlling pairing, Ph2 located on the short arm of 3DS, lacks the 3DS-specific members of the pAWJL3 family bands. The genes are shown to be expressed only after leptotene and predominantly at zygotene and pachytene of meiosis I. The deduced amino acid sequence encoded by the cDNA clone shows two domains, one with three leucine-rich, 24-amino acid repeats and the other with four leucine heptad repeats that resemble those found in basic leucine zipper proteins.

Identification and characterization of stamen- and tapetum-specific genes from tomato

Differential screening of a tomato cDNA library produced from pre-anthesis stamens resulted in the isolation of 25 cDNA clones that hybridized to probes made from stamen RNA and showed no hybridization to probes made from RNA of vegetative organs. The 25 clones were found to represent 11 noncross-hybridizing classes. The majority of these clones were derived from genes that were single or low copy in the tomato genome. Northern RNA blotting experiments of vegetative and floral organs at several stages of development demonstrated that expression in all 11 classes was confined to floral organs. Of the 11 classes 9 were found to be expressed exclusively in stamens prior to anthesis. Two classes showed expression in immature stamens and in petals, with one of these two additionally being expressed in mature stamens at anthesis. Clones from three of the classes that were expressed exclusively in stamens were used as probes for in situ localization of RNA in floral organs. These experiments demonstrated that expression of the genes corresponding to these clones was confined to the tapetal cells of the anthers. Expression of one of the three genes was found to be limited to a single cell type during the 5-6 day period from late meiosis to immature pollen formation.

Meiosis-specific transcripts of a DNA component replicated during chromosome pairing: homology across the phylogenetic spectrum

In meiotic cells of Lilium, a group of single or low copy number DNA sequences that constitute about 0.1-0.2% of the genome do not replicate during the premeiotic S-phase but do so at zygotene in coordination with chromosome pairing. An appreciable fraction of these sequences has now been found to be transcribed into poly(A)+ RNA when chromosomes initiate the pairing process. This "zygRNA" has not been detected in nonmeiotic tissues. Even within the meiocytes, zygRNA is not detectable prior to leptotene or beyond midpachytene. S1 nuclease digestion of mouse spermatocyte nuclei selectively released zygDNA, which hybridizes with lily zygRNA. zygRNA has not been detected in mouse somatic tissues. The profile of zygRNA formation and disappearance in mouse spermatocytes is very similar to that of zygRNA in lily meiocytes.

Replication and nicking of zygotene DNA sequences. Control by a meiosis-specific protein

A protein having a molecular weight of 73,000 daltons has been purified from the nuclear membranes of preleptotene, leptotene, and zygotene cells. It has been named the leptotene protein (L-protein) because of its role in suppressing the replication of zygotene DNA sequences through leptotene until the initiation of zygotene DNA synthesis. The protein has been found to be highly specific in its inhibitory activity. S-phase replication in somatic nuclei and in microspore nuclei are unaffected by the protein. Only zygotene DNA sequences appear to be affected. L-protein binds specifically to zygotene DNA. The binding is limited to a relatively short DNA segment, probably no longer than 90 base pairs (bp). Chloroplast and mitochondrial DNA do not bind to the protein, but a low level of binding is displayed by DNA from several other eukaryotic species. The L-protein also has the capacity to nick the bound DNA in the presence of ATP. Nicking does not occur in the absence of binding. Using supercoiled plasmids with zygotene DNA inserts as substrates, the nicking has been found to be confined to a small region of the plasmid and to occur in only one of the strands. The L-protein is considered to be one of the principal factors responsible for the irreversible commitment of cells to meiosis at the end of the preleptotene S-phase. It is also proposed that its endonucleolytic activity plays a role in the initiation of synapsis.

cDNA clones encoding cytoplasmic poly(A)+ RNAs which first appear at detectable levels in haploid phases of spermatogenesis in the mouse

We have isolated several cDNA clones encoding cytoplasmic poly(A)+ RNAs which are enriched in postmeiotic (haploid) spermatogenic cells in the mouse. Seventeen of 750 clones from a testis cDNA library hybridized more strongly to 32P-labeled cDNA copied from cytoplasmic poly(A) RNA of round spermatids than pachytene spermatocytes. Northern gel blots demonstrated that these 17 plasmids hybridized to RNA(s) approximately 0.5 kb (1 clone), 0.7 kb (13 clones), 0.8 kb (1 clone), and 0.9 kb (2 clones). Four plasmids hybridizing to RNAs 0.7 and 0.9 kb were further characterized by Northern blots. The levels of hybridization were about 10-fold greater with RNA from round spermatids, elongating spermatids and residual bodies than from pachytene spermatocytes from adult testis. These plasmids did not hybridize with cytoplasmic poly(A)+ RNA from sexually immature testis, adult liver, or brain, larger precursors in adult testis nuclear RNA, total RNA from cultured Sertoli cells, poly(A)- RNA from adult testis or the mouse mitochondrial genome. These results demonstrate that certain poly(A)+ RNAs are abundant in haploid cells but barely or not detectable in meiotic cells suggesting the accumulation of these RNAs in round spermatids requires transcription in haploid cells.