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Experimental Hybrids Between Ascidians and Sea Urchins

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Experimental Hybrids Between Ascidians and Sea Urchins Williamson and Boerboom, 1:4 http://dx.doi.org/10.4172/scientificreports.230 Open Access Open Access Scientific Reports Scientific Reports Research Article OpenOpen Access Access Experimental Hybrids between Ascidians and Sea Urchins Donald I Williamson1* and Nicander G J Boerboom2 1School of Biological Sciences, University of Liverpool L69 3BX, UK 2Spechtstraat 9, 6921 KP Duiven, the Netherlands Abstract The larval transfer hypothesis claims that larvae did not evolve gradually from the same stocks as adults but were transferred by hybridization from animals in distantly related taxa. Experimental hybrids between organisms from different phyla demonstrate the feasibility of crossing distantly related animals and they provide material to study the morphologies, chromosomes and genomes of hybrids. Untreated eggs of the ascidian Ascidia mentula mixed with concentrated sperm of the sea urchin Echinus esculentus divided into 33 of 63 experiments. One experiment yielded 3,000 eight-armed pluteus larvae, a minority of which developed into sea urchins. Two pentaradial sea urchins and one tatraradial sea urchin survived more than four years and produced fertile eggs. The majority of the 3,000 plutei resorbed their arms to become spheroids. Forms resembling tadpoles developed within some of these spheroids, but no tadpoles emerged. Eggs of the sea urchin Psammechinus miliaris, pretreated with acid seawater before mixing with dilute sperm of the ascidian Ascidiella aspersa, developed into four-armed pluteus larvae, all of which resorbed their arms to become bottom-living spheroids that divided repeatedly. Some of these spheroids developed inclusions with ascidian features, but they did not develop further. Many spheroids grew into irregular shapes; others divided to produce more spheroids. The results show that, on occasion, some distantly related animals will hybridize in the laboratory and larval and adult forms can retain their identities in hybrids. The findings are consistent with the larval transfer hypothesis. Biologists are urged to conduct comparable hybridization experiments. Keywords: Evolution; Larval transfer; Pluteus larvae; Tadpoles; Self- discussed here are not attempts to replay evolutionary history, but they replicating spheroids demonstrate that interphyletic hybridization can occur. Introduction Brief accounts of the crosses described here as Experiments 1 and 2 are included in two books [2,5], but they lack the experimental details. Most modern biologists follow Darwin in assuming that larvae The present paper includes hitherto unpublished photographs relating and their corresponding adults evolved from common ancestors [1]. to Experiment 2 and it provides evidence that some hybrids developed As a result, animals are frequently separated into protostomes and further than was previously realized. Experiments 3 and 4 have not been deuterostomes by their larvae; molluscs, annelids and several other described previously. The experiments started in 1988-95 (Experiments phyla are thought to have evolved from a common protostome ancestor 1 and 2) were carried out by DIW and both authors collaborated in with trochophore larvae and echinoderms and hemichordates are those started in 2002 (Experiments 3 and 4). thought to have evolved from a common deuterostome ancestor with tornaria larvae. The ‘common ancestor’ hypothesis is, however, difficult Materials and Methods or impossible to reconcile with: We describe the following hybrid crosses: (a) Recently acquired larvae, where animals have apparently suddenly acquired fully evolved larva, as in species of Hebella (a) eggs of the ascidian Ascidia mentula of Müller, 1776 (phylum (Hydrozoa) [2,3]. Chordata, subphylum Urochordata) fertilized with sperm of the sea urchin Echinus esculentus Linnaeus, 1758 (phylum Echinodermata, (b) Incongruous larvae, where the larva seems to be only distantly Class Echinomorpha) related to the adult, as in Hebella (Hydrozoa), Dromiidae (Crustacea, Brachyura) and Rhizocephala [2,4-6]. (b) eggs of the ascidian Ciona intestinalis (Linnaeus, 1767) fertilized with sperm of the sea urchin Psammechinus miliaris (Gmelin, 1778) (c) Multiple larvae in the same life history, as in Penaeidae (Crustacea) and some insects [2,5]. (c) eggs of P. miliaris fertilized with sperm of the ascidian Ascidiella aspersa (OF Müller, 1776). We discuss the relevance of these crosses (d) ‘Overlapping metamorphosis’, in which the larva and juvenile to the larval transfer hypothesis. Throughout the paper we use the live side by side, as in Luidia sarsi (Echinodermata, Asteromorpha) [2,5]. ‘eggs first’ convention for hybrids. eTh crosses described are, therefore, (e) ‘Start again metamorphosis’, in which no larval tissues or organs survive metamorphosis, as in bryozoans with trochophore or *Corresponding author: D I Williamson, School of Biological Sciences, University cyphonautes larvae and holometabolous insects [2-4,6]. of Liverpool, 14 Pairk Beg, Port Erin, Isle of Man IM9 6NH, UK, Tel: 44 1624 832625; E-mail: [email protected] These and other examples are discussed fully in previous Received July 11, 2012; Published July 14, 2012 publications and larval transfer is proffered as an alternative explanation [2-6]. Larval transfer claims that the basic forms of all larvae were later Citation: Williamson DI, Boerboom NGJ (2012) Experimental Hybrids between Ascidians and Sea Urchins. 1: 233. doi:10.4172/scientificreports.230 additions to animal life histories. They originated as adults in other taxa and their genomes were transferred by hybridization. Experimental Copyright: © 2012 Williamson DI, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits un- hybrids test the feasibility of crossing distantly related animals and they restricted use, distribution, and reproduction in any medium, provided the original provide material to study the fate of genes in hybrids. The experiments author and source are credited. Volume 1 • Issue 4 • 2012 Citation: Williamson DI, Boerboom NGJ (2012) Experimental Hybrids between Ascidians and Sea Urchins. 1: 233. doi:10.4172/scientificreports.230 Page 2 of 7 Ascidia mentula x Echinus esculentus, Ciona intestinalis x Psammechinus no eggs divided. Experiments 1 and 2 (described below) are cases in miliaris and Psammechinus miliaris x Ascidiella aspersa. which more than 100 A. mentula eggs hatched as ciliated blastulas after exposure to E. esculentus sperm. The work took place at Port Erin Marine Laboratory (University of Liverpool), Port Erin, Isle of Man, UK, which closed in 2006. Control for experiment 1, started 27 March 1989: Echinus x Filtered seawater used in all experiments had been passed through Echinus a 0.1 μm filter. Some bottom-living hybrids from the 2002 experiments 27 March 1989. Several newly collected specimens of Echinus were kept in running seawater passed through a 1.0 μm filter. Port Erin esculentus started to spawn spontaneously when placed in seawater. seawater was at pH 8.4. Acid seawater was prepared by gradually adding Gametes were collected from three males and one female. One drop concentrated acetic acid to filtered seawater until the pH fell to 5.0 [7]. of sperm was mixed with about 300 eggs in a litre of seawater in a Specimens of Ascidia mentula used in the 1989-90 experiments were homospermic fertilization. The remainder of the sperm was stored at obtained from an old covered seawater storage tank at Port Erin Marine 3-5°C until the next day. Laboratory. This tank was drained soon after the 1990 experiments and 28 March: at 10-12°C, 99% of eggs hatched as ciliated blastulas, 27 never refilled. After 1990, divers obtained specimens of A. mentula from a shipwreck, five km northwest of Port Erin. Specimens of Ciona hours from fertilization. 29 March: larvae gastrulated by invagination, intestinalis, Ascidiella aspersa and other ascidians used in 2002 were then became prism-shaped. 30 March: Pseudoisochrysis added as obtained from other seawater storage tanks at the Marine Laboratory food. 31 March: mouth formed. 2 April: four-armed plutei. 5 April: and by diving on the ruined breakwater at Port Erin. Divers also Pseudoisochrysis and Chlorella added as food. 9 April: six-armed plutei. collected Echinus esculentus from the breakwater. Psammechinus 12 April: eight-armed plutei. 19 April: anterior epaulettes in some miliaris was obtained from a mid-tide rock pool at Gansey Point, Port larvae. 6 May: one larva with juvenile rudiment. Mortality was high St Mary and Isle of Man. and no free-living juveniles were obtained. In 1989-1995, large specimens of A. mentula were gently squeezed Experiment 1, started 28 March 1989: Ascidia x Echinus. to express eggs or sperm. In 2002, specimens of several species of 28 March 1989: laboratory temperature 10-12°C. 14.00: specimen ascidians were isolated in bowls of filtered seawater. Ripe specimens of Ascidia mentula, gently squeezed, yielded 250 eggs. Eggs washed in shed eggs or sperm, usually overnight. Although ascidians are simultaneous hermaphrodites, they only rarely emit eggs and sperm filtered seawater. None divided over three hours. Seawater drained from together. Eggs were observed at intervals for at least two hours and any eggs. 17.00-17.20: eggs immersed in concentrated sperm from three batch with dividing eggs was discarded. E. esculentus, obtained on 27 March. Eggs then repeatedly washed in filtered seawater to remove excess sperm. Re-count confirmed 250 eggs. Ripe sea urchins will sometimes
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