Cel­lu­lar and De­ve­lo­p­men­tal Bio­lo­gy of Plants

Ase­xu­el­le männ­li­che und weib­li­che Vol­vox-​Sphäroide be­sit­zen den glei­chen Phä­no­typ. Unter Stan­dard­be­din­gun­gen dau­ert der ase­xu­el­le Le­bens­zy­klus 48 h und wird durch einen 18 h-​Hell-8 h-​Dunkel-Zyklus syn­chro­ni­siert. (In der Ab­bil­dung sind je­weils nur vier der sech­zehn re­pro­duk­ti­ven Zel­len bzw. Em­bry­os bzw. Toch­ter­sphä­ro­ide ein­ge­zeich­net). Wäh­rend der Em­bryo­ge­ne­se, die ca. 8 h dau­ert, füh­ren die rei­fen re­pro­duk­ti­ven Zel­len (Go­ni­di­en) eine Serie von Zell­tei­lun­gen durch (11-12 Tei­lun­gen). Eine die­ser Tei­lun­gen ist asym­me­trisch: Die grö­ße­ren Zel­len die aus die­ser asym­me­tri­schen Tei­lung ent­ste­hen wer­den zu den Go­ni­di­en der nächs­ten Ge­nera­ti­on, die klei­ne­ren Zel­len wer­den spä­ter zu den so­ma­ti­schen Zel­len. Am Ende der Tei­lun­gen be­fin­det sich im Em­bryo die spä­te­re In­nen­sei­te außen und um­ge­kehrt, d.h. die Go­ni­di­en lie­gen außen und die Fla­gel­len­an­sät­ze der so­ma­ti­schen Zel­len zei­gen zum In­ne­ren der Hohl­ku­gel. Der gastru­la­ti­ons­ähn­li­che Pro­zeß der In­ver­si­on, der am Ende der Em­bryo­ge­ne­se statt­fin­det, bringt den Em­bryo in die Kon­fi­gu­ra­ti­on der er­wach­se­nen Algen. An­schlie­ßend  ex­pan­die­ren die Em­bry­os, bzw. jetzt Toch­ter­sphä­ro­ide ge­nannt, durch den Auf­bau der ex­tra­zel­lu­lä­ren Ma­trix und schlüp­fen aus dem el­ter­li­chen Sphä­ro­id (="Re­lease"). Der leere el­ter­li­che Sphä­ro­id be­steht nur aus so­ma­ti­schen Zel­len, die nicht in der Lage sind sich wei­ter zu tei­len. Diese so­ma­ti­schen Zel­len al­tern und ster­ben ab. Un­ter­des­sen rei­fen die Go­ni­di­en der frei­ge­setz­ten Toch­ter­ko­lo­nien und be­gin­nen wie­der mit den Tei­lun­gen.

Vol­vox ist in der Lage sich so­wohl ase­xu­ell als auch se­xu­ell zu ver­meh­ren. Die se­xu­el­le Ver­meh­rung wird durch ein Glykoprotein-​Pheromon ein­ge­lei­tet, das bis zu einer Kon­zen­tra­ti­on von ca. 10^-16 M (!!!) volle bio­lo­gi­sche Ak­ti­vi­tät zeigt. Somit kann ein ein­zi­ges se­xu­el­les Männ­chen ge­nü­gend Sex-​induzierendes Phe­ro­mon pro­du­zie­ren um viele Mil­lio­nen ase­xu­el­ler weib­li­cher Sphä­ro­ide in den se­xu­el­len Zy­klus zu brin­gen. Das Sex-​induzierende Phe­ro­mon be­wirkt, daß sich das Tei­lungs­mus­ter einer re­pro­duk­ti­ven Zelle (Go­ni­di­um) än­dert. Im ase­xu­el­len Zy­klus von weib­li­chen und männ­li­chen Vol­vox fin­det die erste asym­me­tri­sche Tei­lung bei 16 Zel­len im 32-​Zell-Embryo statt, wo­durch 16 Zel­len ent­ste­hen, die sich spä­ter zu Go­ni­di­en ent­wi­ckeln. In Go­ni­di­en von weib­li­chen Sphä­ro­iden mit Pheromon-​Kontakt ist die erste asym­me­tri­sche Tei­lung um eine Runde ver­zö­gert, wo­durch 32 Zel­len ent­ste­hen, die sich spä­ter zu den 32 Eiern ent­wi­ckeln. Bei Go­ni­di­en von männ­li­chen Sphä­ro­iden mit Pheromon-​Kontakt fin­det die erste asym­me­tri­sche Tei­lung erst im 256-​Zell-Embryo statt, wo­durch so­ma­ti­sche und re­pro­duk­ti­ve (so­ge­nann­te "An­dro­go­ni­di­en") Zel­len im Ver­hält­nis 1:1 ent­ste­hen. Die An­dro­go­ni­di­en ent­wi­ckeln sich spä­ter zu Pa­ke­ten aus be­gei­ßel­ten Sper­mi­en­zel­len.

Sche­ma­ti­scher Quer­schnitt durch einen Vol­vox-​Sphäroid unter be­son­de­rer Be­rück­sich­ti­gung der ex­tra­zel­lu­lä­ren Ma­trix. Die ex­tra­zel­lu­lä­re Ma­trix von Vol­vox ist in vier Haupt­zo­nen un­ter­teilt (vgl. Kirk et al., 1986; J. Cell Sci. 80, 207-​231): CZ, "cel­lu­lar zone", Zell-​Zone; DZ, "deep zone", Innen-​Zone; FZ, "bounda­ry zone", Grenz-​Zone; FZ, "fla­gel­lar zone", Flagellen-​Zone.

Di­vi­si­on sta­ges du­ring em­bryo­ge­ne­sis of Vol­vox car­te­ri. The first five clea­va­ge sta­ges in the de­ve­lo­p­ment from a sin­gle re­pro­duc­ti­ve cell to a 32-​celled em­bryo are com­pa­red. D. Vol­vox car­te­ri: D a, high­ly va­cuo­la­ted go­ni­di­um prior to the onset of clea­va­ge; D b, two-​cell em­bryo; va­cuo­les have disap­peared, the first clea­va­ge fur­row is vi­si­ble and also the ve­si­cle has be­co­me ap­pa­rent; D c, four-​cell em­bryo; D d, eight-​cell em­bryo; there are two stag­ge­red rings with four cells each; the pore in the cent­re is the phialo­po­re; D e, sixteen-​cell em­bryo, D f, thirty-​two-cell em­bryo. Scale bars are given on all pic­tu­res, and their lengths are given on the first pic­tu­re.

Wäh­rend der Em­bryo­nal­ent­wick­lung von Vol­vox er­ge­ben elf oder zwölf schnel­le und syn­chro­ne Zell­tei­lun­gen einer re­pro­duk­ti­ven Zelle (Go­ni­di­um) sämt­li­che Zel­len eines er­wach­se­nen Or­ga­nis­mus. Die ent­ste­hen­den Em­bryo­nal­zel­len sind be­reits als Hohl­ku­gel an­ge­ord­net, aber ihre Ori­en­tie­rung be­züg­lich der Ku­gel­ober­flä­che ist ent­ge­gen­ge­setzt zu der die man in einem er­wach­se­nen Or­ga­nis­mus fin­det: Die Zel­len­den mit den spä­te­ren Fla­gel­len zei­gen zum Zen­trum der Kugel und die re­pro­duk­ti­ven Zel­len (Go­ni­di­en) ste­hen von der Ober­flä­che ab. Wäh­rend der In­ver­si­on stülpt sich der Em­bryo kom­plett um (durch einen kreuz­för­mi­gen Schlitz, die "Phialo­po­re"), wo­durch er die Ori­en­tie­rung der er­wach­se­nen Or­ga­nis­men er­hält. Die In­ver­si­on er­gibt sich aus einer Folge von Zell­form­än­de­run­gen, die sich wel­len­ar­tig von der Phialo­po­re aus­ge­hend zum ent­ge­gen­ge­setz­ten Pol aus­brei­ten (siehe Ab­bil­dun­gen von oben nach unten).

In­ver­si­on in Vol­vox car­te­ri. A-H: Suc­ces­si­ve steps of in­ver­si­on. Scale bars = 25 µm.

Sty­li­zed se­quence of the in­ver­si­on pro­cess in Vol­vox car­te­ri. Cross-​sections are shown. In­ver­si­on hap­pens along the anterior-​posterior axis of each or­ga­nism and the anterior-​posterior axis wit­hin this fi­gu­re is par­al­lel to the prin­ted page. At the be­gin­ning of in­ver­si­on, the an­te­rior pole is at the top, after in­ver­si­on the pole is on the bot­tom, but now (by con­ven­ti­on) it has be­co­me the pos­te­rior pole of the in­ver­ted em­bryo. The side of the cell layer that is out­si­de in the adult con­fi­gu­ra­ti­on and from which the fla­gel­la will emer­ge is given in black. Areas and di­rec­tions of cell layer mo­vement are in­di­ca­ted by small black ar­rows.

Di­vi­si­on sta­ges du­ring em­bryo­ge­ne­sis of Go­ni­um pec­to­ra­le. The first five clea­va­ge sta­ges in the de­ve­lo­p­ment from a sin­gle re­pro­duc­ti­ve cell to a 16-​celled em­bryo are com­pa­red. A. Go­ni­um pec­to­ra­le. A a: cell prior to the onset of clea­va­ge; A b-e: in 4 lon­gi­tu­di­nal cell clea­va­ges a 16-​cell pla­kea is for­med; A f: side view of the 16-​cell pla­kea sho­wing that it is stron­gly cur­ved. Scale bars are given on all pic­tu­res, and their lengths are given on the first pic­tu­re.

In­ver­si­on in Go­ni­um pec­to­ra­le. A-D: Suc­ces­si­ve steps of in­ver­si­on. Scale bars = 25 µm.

Sty­li­zed se­quence of the in­ver­si­on pro­cess in Go­ni­um pec­to­ra­le. Cross-​sections are shown. In­ver­si­on hap­pens along the anterior-​posterior axis of each or­ga­nism and the anterior-​posterior axis wit­hin this fi­gu­re is par­al­lel to the prin­ted page. At the be­gin­ning of in­ver­si­on, the an­te­rior pole is at the top, after in­ver­si­on the pole is on the bot­tom, but now (by con­ven­ti­on) it has be­co­me the pos­te­rior pole of the in­ver­ted em­bryo. The side of the cell layer that is out­si­de in the adult con­fi­gu­ra­ti­on and from which the fla­gel­la will emer­ge is given in black. Areas and di­rec­tions of cell layer mo­vement are in­di­ca­ted by small black ar­rows.

Di­vi­si­on sta­ges du­ring em­bryo­ge­ne­sis of Pan­do­ri­na morum. The first five clea­va­ge sta­ges in the de­ve­lo­p­ment from a sin­gle re­pro­duc­ti­ve cell to a 32-​celled em­bryo are com­pa­red. B. Pan­do­ri­na morum. B a: cell prior to the onset of clea­va­ge; B b-e: in 4 lon­gi­tu­di­nal cell clea­va­ges a 16-​cell pla­kea is for­med; Bf: side view of the 16-​cell pla­kea sho­wing that it is cur­ved. Scale bars are given on all pic­tu­res, and their lengths are given on the first pic­tu­re.

In­ver­si­on in Pan­do­ri­na morum. A-H: Suc­ces­si­ve steps of in­ver­si­on. Scale bars = 25 µm.

Sty­li­zed se­quence of the in­ver­si­on pro­cess in Pan­do­ri­na morum. Cross-​sections are shown. In­ver­si­on hap­pens along the anterior-​posterior axis of each or­ga­nism and the anterior-​posterior axis wit­hin this fi­gu­re is par­al­lel to the prin­ted page. At the be­gin­ning of in­ver­si­on, the an­te­rior pole is at the top, after in­ver­si­on the pole is on the bot­tom, but now (by con­ven­ti­on) it has be­co­me the pos­te­rior pole of the in­ver­ted em­bryo. The side of the cell layer that is out­si­de in the adult con­fi­gu­ra­ti­on and from which the fla­gel­la will emer­ge is given in black. Areas and di­rec­tions of cell layer mo­vement are in­di­ca­ted by small black ar­rows.

Di­vi­si­on sta­ges du­ring em­bryo­ge­ne­sis of Eu­do­ri­na uni­coc­ca. The first five clea­va­ge sta­ges in the de­ve­lo­p­ment from a sin­gle re­pro­duc­ti­ve cell to a 32-​celled em­bryo are com­pa­red. C. Eu­do­ri­na uni­coc­ca. C a: cell prior to the onset of clea­va­ge; C b-f: in 5 cell clea­va­ges a 32-​cell cur­ved pla­kea is for­med. Scale bars are given on all pic­tu­res, and their lengths are given on the first pic­tur

In­ver­si­on in Eu­do­ri­na uni­coc­ca. A-H: Suc­ces­si­ve steps of in­ver­si­on. Scale bars = 25 µm.

Sty­li­zed se­quence of the in­ver­si­on pro­cess in Eu­do­ri­na uni­coc­ca. Cross-​sections are shown. In­ver­si­on hap­pens along the anterior-​posterior axis of each or­ga­nism and the anterior-​posterior axis wit­hin this fi­gu­re is par­al­lel to the prin­ted page. At the be­gin­ning of in­ver­si­on, the an­te­rior pole is at the top, after in­ver­si­on the pole is on the bot­tom, but now (by con­ven­ti­on) it has be­co­me the pos­te­rior pole of the in­ver­ted em­bryo. The side of the cell layer that is out­si­de in the adult con­fi­gu­ra­ti­on and from which the fla­gel­la will emer­ge is given in black. Areas and di­rec­tions of cell layer mo­vement are in­di­ca­ted by small black ar­rows.

Di­vi­si­on sta­ges du­ring em­bryo­ge­ne­sis of Vol­vox ter­ti­us. The first five clea­va­ge sta­ges in the de­ve­lo­p­ment from a sin­gle re­pro­duc­ti­ve cell to a 32-​celled em­bryo are com­pa­red. E. Vol­vox ter­ti­us. E a, va­cuo­la­ted go­ni­di­um prior to the onset of clea­va­ge; the ve­si­cle al­rea­dy de­tached from the go­ni­di­um; E b, two-​cell em­bryo; E c, four-​cell em­bryo; E d, eight-​cell em­bryo; E e, sixteen-​cell em­bryo, E f, thirty-​two-cell em­bryo. Scale bars are given on all pic­tu­res, and their lengths are given on the first pic­tu­re.

In­ver­si­on in Vol­vox ter­ti­us. A-H: Suc­ces­si­ve steps of in­ver­si­on. Scale bars = 25 µm.

Sty­li­zed se­quence of the in­ver­si­on pro­cess in Vol­vox ter­ti­us. Cross-​sections are shown. In­ver­si­on hap­pens along the anterior-​posterior axis of each or­ga­nism and the anterior-​posterior axis wit­hin this fi­gu­re is par­al­lel to the prin­ted page. At the be­gin­ning of in­ver­si­on, the an­te­rior pole is at the top, after in­ver­si­on the pole is on the bot­tom, but now (by con­ven­ti­on) it has be­co­me the pos­te­rior pole of the in­ver­ted em­bryo. The side of the cell layer that is out­si­de in the adult con­fi­gu­ra­ti­on and from which the fla­gel­la will emer­ge is given in black. Areas and di­rec­tions of cell layer mo­vement are in­di­ca­ted by small black ar­rows.

Di­vi­si­on sta­ges du­ring em­bryo­ge­ne­sis of Vol­vox glo­ba­tor. The first five clea­va­ge sta­ges in the de­ve­lo­p­ment from a sin­gle re­pro­duc­ti­ve cell to a 32-​celled em­bryo are com­pa­red. F. Vol­vox glo­ba­tor. F a, va­cuo­la­ted go­ni­di­um prior to the onset of clea­va­ge (sur­roun­ded by so­ma­tic cells); F b, two-​cell em­bryo; F c, four-​cell em­bryo; F d, eight-​cell em­bryo; F e, sixteen-​cell em­bryo, F f, thirty-​two-cell em­bryo. Scale bars are given on all pic­tu­res, and their lengths are given on the first pic­tu­re.

In­ver­si­on in Vol­vox glo­ba­tor. A-H: Suc­ces­si­ve steps of in­ver­si­on. Scale bars = 25 µm.

Sty­li­zed se­quence of the in­ver­si­on pro­cess in Vol­vox glo­ba­tor. Cross-​sections are shown. In­ver­si­on hap­pens along the anterior-​posterior axis of each or­ga­nism and the anterior-​posterior axis wit­hin this fi­gu­re is par­al­lel to the prin­ted page. At the be­gin­ning of in­ver­si­on, the an­te­rior pole is at the top, after in­ver­si­on the pole is on the bot­tom, but now (by con­ven­ti­on) it has be­co­me the pos­te­rior pole of the in­ver­ted em­bryo. The side of the cell layer that is out­si­de in the adult con­fi­gu­ra­ti­on and from which the fla­gel­la will emer­ge is given in black. Areas and di­rec­tions of cell layer mo­vement are in­di­ca­ted by small black ar­rows.