student profile: Mr Mohammad Sadequr Rahman Khan


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Thesis work

Thesis title: Evolution of brooding and viviparity in marine invertebrates

Supervisors: Michael THOMPSON , Maria BYRNE , Camilla WHITTINGTON

Thesis abstract:

Matrotrophic brooding in aplacental viviparous animals involves a diverse range of maternal-fetal relationships in different phyla and includes intragonadal lecithotrophic brooding in the Echinodermata. The evolution of viviparity requires internal fertilization and retention of offspring, but the evolution of matrotrophy requires extraembryonic nutrition to be provided to offspring within mother’s body by adopting specialized physiological and morphological mechanisms. Therefore, the brooding strategy and nutrient transfer responsible for advanced brooding require a thorough investigation to understand the evolution of reproductive strategies in invertebrates. I will describe the brooding mechanisms, gonadal morphological changes, and molecules involved in the evolution of viviparous matrotrophic and broadcast spawning echinoderms to understand the morphological changes associated with viviparity in sea star. In my PhD project, I aim to identify the brooding pattern of viviparous sea stars of Australia, investigate maternal-fetal relationships evolved in an intragonadal viviparous matrotrophic invertebrate. Three brooding viviparous species Parvulastra vivipara, P. parvivipara and Cryptasterina hystera have been collected and their contrasting brooding and maternal allocation strategy have been studied through analyzing maternal correlation and offspring size variation. Advanced non-invasive imaging techniques (μCT), and confocal microscopy have been applied to investigate brood arrangement pattern in matrotrophic P. parvivipara. Brood arrangement has been characterized by using micro-computed tomography (μCT) of the whole sea star, which has previously been applied successfully to study echinoderms in situ. Juveniles show a characteristic defensive pattern of offspring arrangement by positioning their oral surface opposite to each other presumable to cannibalize siblings or to protect themselves from being eaten. Laboratory observation proves that the parent may retain juveniles larger than the released juveniles and do not die soon after birth. Any morphological modification in the gonadal epithelial cell that may be required for viviparous larval development will be studied using Transmission Electron Microscopy. I will also locate membrane molecules in the gonad through enzyme histochemistry to explain possible nutrient transfer mechanism in P. parvivipara. I will compare viviparous and oviparous congeners to determine whether similar mechanisms are responsible for the evolution of viviparity and matrotrophy in other invertebrates

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