Embryology and development of a freshwater prosobranch,Melania scabra

Embryology and development of the snail, Melania scabra was studied. The embryos complete their development in a special incubatory pouch called the broodpouch and hatch out in a form completely resembling that of the adult. The embryonic development of the snail represents specialized features like precocious development of the statocyst, presence of a well developed velar retractor muscle and torsion.     

Identification of de novo copy number variants associated with human disorders of sexual development

Disorders of sexual development (DSD), ranging in severity from genital abnormalities to complete sex reversal, are among the most common human birth defects with incidence rates reaching almost 3%. Although causative alterations in key genes controlling gonad development have been identified, the majority of DSD cases remain unexplained. To improve the diagnosis, we screened 116 children born with idiopathic DSD using a clinically validated array-based comparative genomic hybridization platform. 8951 controls without urogenital defects were used to compare with our cohort of affected patients. Clinically relevant imbalances were found in 21.5% of the analyzed patients. Most anomalies (74.2%) evaded detection by the routinely ordered karyotype and were scattered across the genome in gene-enriched subtelomeric loci. Among these defects, confirmed de novo duplication and deletion events were noted on 1p36.33, 9p24.3 and 19q12-q13.11 for ambiguous genitalia, 10p14 and Xq28 for cryptorchidism and 12p13 and 16p11.2 for hypospadias. These variants were significantly associated with genitourinary defects (P = 6.08×10⁻¹²). The causality of defects observed in 5p15.3, 9p24.3, 22q12.1 and Xq28 was supported by the presence of overlapping chromosomal rearrangements in several unrelated patients. In addition to known gonad determining genes including SRY and DMRT1, novel candidate genes such as FGFR2, KANK1, ADCY2 and ZEB2 were encompassed. The identification of risk germline rearrangements for urogenital birth defects may impact diagnosis and genetic counseling and contribute to the elucidation of the molecular mechanisms underlying the pathogenesis of human sexual development.     

Altering sexual development inArabidopsis

The reproductive system determines the way in which gametes develop and interact to form a new organism. Therefore, it exerts the primary level of control of genotypic frequencies in plant populations, and plays a fundamental role in plant breeding. A basic understanding of plant reproductive development will completely transform current breeding strategies used for seed production. Apomixis is an asexual form of reproduction in which embryogenesis occurs in a cell lineage lacking both meiosis and fertilization, and that culminates in the formation of viable progeny genetically identical to the mother plant. The transfer of apomixis into sexual crops will allow the production of self-perpetuating improved hybrids, and the fixation of any desired heterozygous genotype. The initiation of apomictic development invariably takes place at early stages of ovule ontogeny, before the establishment of the megagametophytic phase. The developmental versatility associated with megagametophyte formation suggests that the genetic and molecular regulation of apomixis is intimately related to the regulation of sexuality. Differences between the initiation of sexual and apomictic development may be determined by regulatory genes that act during megasporogenesis, and that control events leading to the formation of unreduced female gametophytes. To test this hypothesis, we are isolating and characterizing genes that act during megasporogenesis inArabidopsis thaliana and investigating their potential role in the induction of apomixis. We are using a recently established transposon-based enhancer detection and gene trap insertional mutagenesis system that allows the identification of genes based on their expression patterns. An initial screen of transposants has yielded over 20 lines conferring restricted GUS expression during early ovule development. We have obtained the sequence of genomic fragments flanking the transposon insertion. Several have homology to genes playing important roles in plant and animal development. They include cell cycle regulators, enzymes involved in callose hydrolysis, leucine-rich repeat protein kinase receptors, and expressed sequence tags (ESTs) of unknown function. Independently, a genetic screen allows the identification of female sterile mutants defective in megasporogenesis. Results from these experiments will improve our basic understanding of reproductive development in plants, and will set the basis for a sustained effort in plant germ line biotechnology, a first step toward a flexible transfer of apomixis into a large variety of sexual crops.     

Triggers and tricks of Plasmodium sexual development

Irrespective of the tremendous suffering caused by malaria, a Plasmodium infection by pathogenic blood stages is only transient and an obligate step toward the Anopheles vector where sexual reproduction and genetic recombination of the unicellular parasite takes place. Recent expression profiling studies identified the molecular make-up of female and male gametocytes. Differential promoters and translational repression through mRNA binding by a female-specific helicase help to fine-tune the expression of these sexual stage-specific genes. However, we are only just beginning to discover the triggers that initiate gametocytogenesis and the developmental programs that drive sexual development.     

Embryology of Phoronida

Fertilization in the Phoronida appears to be internal. Threedifferent types of eggs were found: (1) Eggs rich in yolk, about125µ in diameter, which are retained in the parent's tube,without a true pelagic life; (2) Eggs moderately rich in yolk,about 100 µ in diameter, brooded up to the actinotrochstage in the lophophoral concavity owing to the nidamental glands,with a more or less long pelagic life; (3) Eggs, yolk-poor,about 60 µ. in diameter, which are directly dischargedinto sea-water and which have a long pelagic life. Cleavage in the Phoronida is total, equal or subequal. The patternis typically radial though biradial in some stages, but thereare instances in which the blastomeres exhibit a spiral appearance. The gastrula arises generally by emboly. The blastocoel is extensivein embryos of type 3 (see above) and virtually obliterated bywall compression in type 2. The blastopore is reduced to ananterior remnant. The differentiation of the ectoderm leads to the formation ofthe preoral lobe, the apical plate, the tentacular ridge, thenephridial anlage, the oesophagus (issued from the posteriorpart of the vestibule) and the mouth which does not originateas a stomodeum; the blastopore is located between oesophagusand stomach. Differentiation of the archenteron (endoderm) produces the stomach,the intestine and the anus which opens by perforation of theectoderm, without formation of a proctodeum. The anus appearsto be independent of the blastopore. The mesodern originates as isolated cells proliferated fromthe anterior and ventrolateral areas of the archenteron, intwo phases. The mesoderm is formed in a modified enterocoelousmanner. The protocoel is produced first from the anterior archentericwall and occupies the cavity of the preoral lobe; the metacoeloriginates from the ventrolateral mesodermal proliferations.The mode of formation of these cavities seems to vary with species.